Gear Rifles - design assistance requested
 

This is an idea I had (or, rather, shamelessly stole from this thread), for a "rifle" that stored its motive power in a clockwork battery. A good option for how it turns energy stored in said battery into propelling a bullet into some monster's skull, however, has always eluded me.

Until now.

Thanks to the Forgotten Weapons interview with the cofounder of ArcLabs about their GR-1 Anvil Gauss Rifle, I realized I could use a mechanism that pulled the bullet, rather than pushing it, down the barrel. This is what I've come up with. I'll note this is meant for DF, so strict realism is not necessary, although some nods to it would be welcome.

Aesthetically, the weapon would look more-or-less like a typical TL 4-5 musket/rifle, but with an ovoid barrel rather than a circular one (or a rectangle rather than a square, or an elongated hexagon rather than a regular hexagon). Internally, there is a winged "cup" at the back of the barrel, with a hole in top (matching up with one in the barrel; the latter would have a movable cover) for putting the bullet in. Threaded through the wings is a chain on each side, which wraps around and links to itself (going through the elongated portion of the barrel). When the weapon is fired, gears spin to move this chain, in turn accelerating the cup (and bullet within). When the cup reaches the end of the barrel, it strikes a brake (probably made of soft leather, or cork wood), which has a hole in the center for the bullet to pass through - this stops the cup (which slips from the chain on impact, so that the gears can continue turning for a bit rather than coming to a jarring stop; basically, the chain fits it snugly enough that it can be accelerated fairly quickly, but not enough to hold on at the sudden stop at the end). To reload, you flip a safety switch to disengage the gears from the battery, pull the cup back into loading position (it has a tab on it that comes up through a slit in the top of the barrel, and serves as rear sights; for long barrels, it typically has a lanyard attached so you can just grab and yank that back), drop a bullet in, then attach a hand crank, wind the mechanism, detach the hand crank, and finally flip the safety switch back to shooting position while shouldering the weapon.

However, that requires a sufficiently-long reload process that it may reduce the weapons to "one shot per battle," which is very much not what I want. Absent Fast-Draw, and ignoring the time to actually wind the mechanism, I'm thinking you could flip the safety and pull the cup back into position as a single Ready; draw a bullet from your pouch as a Ready; remove the cover, drop the ball into the cup, and replace the cover as a Ready; draw the handcrank as a Ready; attached the handcrank as a Ready; detach the handcrank as a Ready; stow the handcrank as a Ready (you could skip this step if you have it on a lanyard, simply dropping it as a free action); and take a final Ready to, well, Ready the weapon and flip the safety. That's 8 seconds!

What options would I have to speed this up? Having the crank permanently attached would cut time in half, but absolutely ruins the aesthetics of the weapon, so I'd rather avoid that. Might it be feasible to have a folding crank, such that the weapon doesn't have a big bulky thing off to the side normally, but you can pop it out, crank it, and put it back in (with the crank's position serving as the saftey - pulling it out switches the battery to be connected to the winding mechanism, putting it back in switches the battery to be connected to the gears). Or should I allow the battery to store multiple shots, so that reloading during combat only consists of pulling the cup back into position and dropping a bullet in?

For cranking and damage, my inclination is to define battery capacity in terms of pounds, have a character charge it by BL lb with each Ready (either 1.5xBL or 2xBL with an All Out Ready), and have the damage be equal to thr at the ST for which the battery's capacity matches its BL. That is, a battery that can store 80 lb would take a character with ST 10 around four seconds to charge (three or two seconds if going All Out) and deal damage equal to thr at ST 20 (2d-1). Does that sound feasible? Note if you dislike setting battery capacity in terms of pounds, you could instead use Joules - for 5xBL Joules, 7xBL Joules, and 10xBL Joules, respectively (of course, damage probably is unlikely to match up at all using Doug's spreadsheet, but that's fine - this is for Dungeon Fantasy, not Tactical Shooting).

I could go on, but ultimately, these are my current questions (repeating some from above):

1. What are some good options to speed up the loading process? Does a built-in crank make sense? Should storing multiple shots worth of energy be an option, or would we be stepping on the toes of archers too much?
2. Does my BL/cranking time to damage relationship make sense, from a game balance perspective?
3. What sort of weight should these weapons have? I'm thinking of taking the simplified formulae for firearms I worked out based on Classic Vehicles and simply applying a multiplier - that is, a gear rifle would weigh something like 1.2x as much as a comparable firearm.
4. What effect should barrel length have? I'm thinking a longer barrel means the battery can discharge over a longer period of time, increasing efficiency. But by how much - and what should be the "zero point?" Perhaps a "typical" gear rifle is BL 80 and has a 1-yard-long barrel, and weaker weapons can get away with a shorter barrel and still have maximum efficiency, while stronger weapons need a longer one.
5. What about rifling? I like the idea of actual rifled weapons having their brake located partway down the barrel, with a portion of it that only the bullet travels through being rifled, imparting spin (thus, you're basically sacrificing damage to get higher accuracy). Problem is, I want bullets that are made of fired clay (including hollow ones that you can fill with stuff like alchemist's fire) in addition to the typical lead ones, but I don't think said clay would engage with the rifling the same way lead would. Do clay bullets require a smoothbore weapon - or might a wax "jacket" be an option?
6. Caliber. Should the above be for pi, pi+, pi++... or even pi-? How should things scale with other calibers of bullet? One idea might be that the reduced efficiency of having the battery discharge rapidly has to do with the speed of the gears more than the time period over which the energy is discharged, so heavier bullets can get away with shorter barrels to reach maximum efficiency. Even so, what about damage? Should I just scale this like Innate Attack does - that is, if the base is pi, then pi- would be +67% damage (around +2 per die), pi+ would be -17% damage (around -0.5 per die), and pi++ would be -37% damage (around -1.5 per die)?

Any assistance would be greatly appreciated. Hopefully this time I've avoided my chronic issue of "information overload in the first post."

Re: Gear Rifles - design assistance requested
 

I suggest looking up the slingatron concept (originally proposed as a space launch, but people have built them as hobby projects).

Re: Gear Rifles - design assistance requested
 

So my take on this from a physicist's perspective, while not being completely realistic is it sounds very fun and cool. My answers would be:

1. I'd say you should store multiple shots per crank session (maybe 4-6) then have a lengthy winding session. Additionally, you could see a slight reduction in power per additional shot as the gears wind down.

2. I think your cranking set sounds like a good balance between realism and game balance.

3. I think you've got them a bit too light. All this mechanism is going to be far heavier than real guns. I'd say a minimum should be double a comparative TL4 gun.

4. Barrel length I feel is your trickiest and potentially least realistic point. A longer barrel is going to have far more friction with this system than a bullet so I don't think things scaling like a real gun is appropriate. At best you might break even with losses and gains. The most realistic method might be what you suggest with the length being related to the strength of the gun, but I would say if anything the stronger the gun, the shorter the range should be due to friction losses. And it might be interesting effects on usability.

5. Rifling again would be very tricky. Your ideas though are a pretty cool way of giving some rational explanation. I think the wax jacket is most realistic. The trade off of range for accuracy seems most feasible.

6. Caliber seems good the way you've described. Flechette darts might be good too as an option.

Re: Gear Rifles - design assistance requested
 

Fascinating, but I want rifle-shaped weapons, not large discs with spiral grooves in them, which is what it appears slingatrons generally are (Joerg Sprave's version, because of course he made one). This may well be something that is extant elsewhere, however - perhaps stationary defenses often use such a scheme, provided it's more efficient than the gear rifle (and I assume it is - otherwise you'd have people building something more like a gear rifle).

However, the description I first read about it gave me another idea for imparting spin to a projectile. I've heard before that vibrations can cause screws to, well, unscrew themselves - here's a video, ignore the (I think accidentally, but my volume is off) salacious title, they just use a vibrating toothbrush and electric shaver. I'm pretty certain that's an aspect of their threads, but would it be feasible that a vibrating gear rifle - say, because the gears aren't quite properly well-aligned - could impart spin to the projectile? I could see this being an accidental discovery in the early days of replicating the gear rifles found in the dungeon*, where it turned out that getting proper alignment to reduce vibration actually reduced accuracy. So, designers have the choice between better accuracy but lower efficiency (vibration is going to eat up some of your energy), or lower accuracy but better efficiency, getting the sort of accuracy-vs-damage trade-off I'd prefer. But I don't know if that's even remotely realistic.

*Oubliette is roughly TL 2-3 generally, but has found - and successfully replicated - TL 4+ technology in the dungeons that tend to randomly pop up all over the place.

Re: Gear Rifles - design assistance requested
 

I'd suggest having a weapon that takes minutes to wind up, but powers an endless chain (like a chainsaw) that runs for 4-6 seconds.

You drop your cups onto that when you pull the trigger and they get flung downrange. You need a magazine in the weapon, to make this work. That gives you RoF 1 if your magazine is operated automatically, or RoF 1/2 if it behaves more like a bolt-action rifle.

Re: Gear Rifles - design assistance requested
 

For firearms, I'm fond of the idea of a 5-shot revolver at low TL's (because 5 shots is how many you get before black powder fouling comes into play; also, IIRC, in Classic Vehicles, a 5-shot revolver ends up weighing exactly twice as much as a normal breechloader). So being able to hold up to 5 shots could certainly be an option. I'm thinking soldiers typically have single-shot versions, but adventurers are going to spring for the more expensive (and heavier) version.

Damage slowly going down matches the behavior of the TL 5 air rifle in HT, IIRC. I'd have to decide if I want that for DF - I'm honestly leaning toward "no" ("It's your last shot... and it does half damage" seems like adding insult to injury).

Thanks!

Double was actually my first instinct, but I feared that would be a bit too much - in settings where both are available, I want the two to be competitive with each other.

The bullet - and cup for that matter - don't have much direct interaction with the barrel, although I could see a longer chain causing more friction losses. I'm willing to step away from realism here to get the effect I want - namely, that longer barrels (resulting in worse Bulk and higher weight) result in higher damage.

If vibration doesn't work, I think the rifled end of the barrel may well be my best bet.

Flechette darts are an interesting idea. They'd typically be made of iron/steel rather than lead, making them lighter (so damage somewhere between what lead and clay bullets get), but they'd be eligible to be made with hardened steel, for an armor divisor. Damage type would be impaling, but probably with the same WM as the bullet they replaced, rather than the default x2 (the difference between imp and pi here would be interaction with certain types of DR and IT:DR... assuming I opt to have something that differentiates between the two, anyway). Unlike the spherical bullets, orientation would matter a great deal for these - I'd probably give a penalty to Fast-Draw to load them quickly. They'd also risk damaging the rifling if not used with a smoothbore, but if the wax jacket works for clay bullets, it should work for flechettes as well.

I'd rather have independent shots than something that just flings metal downrange for a bit after pulling the trigger (also, I don't think my cup idea would be compatible, here). A magazine could well be an option for the default version of the weapon, although given the way it would generally need to be set up, it would interfere with aiming (although a bottom-mounted magazine, using springs or a manually-operated lever to push bullets up into the cup like a modern firearm rather than gravity to drop them down like a repeating crossbow, might be doable; it would absolutely require a dedicated weapon, or at least modifying a normal gear rifle).

Re: Gear Rifles - design assistance requested
 

You know, the way the gauss rifle works is that you've got a primary electrical energy store (high storage, but not enough power to directly propel the bullet) that charges a secondary energy store (low storage, but high burst power). The equivalent of this for clockwork would be to use clockwork to charge a device of another type, such as a bow, sling, or spring.

Re: Gear Rifles - design assistance requested
 

The gauss rifle mention was more about giving me the idea of having the projectile be pulled rather than pushed than about trying to mimic a gauss rifle at TL DF. That said, from a certain point of view, that's exactly what's happening here - your primary (mechanical) energy store is the character's muscles, the battery is the secondary energy store that uses its high burst power to propel the bullet. Of course, you could also think of the character's muscles as being equivalent to the power plant that charges the battery, in which case we're missing the equivalent of the capacitors. A stretchy material (as used in slingshots) could work, but I think low TL's are rather lacking in such - they rely instead on bendy materials (like wood), and I'm wanting something that looks like a rifle, rather than an autococking crossbow. Although perhaps something using torsion, with the chains replaced with thick string/thin rope, could be an option? That would essentially be something with two triggers. The first releases the primary energy store to wind up the strings and generate torsion, with the cup locked in place. Once it's done "charging," you pull the trigger to release the cup, and it rockets forward, in turn propelling the bullet. You'd probably need to use the weapon shortly after creating the torsion, so gear rifles would be a bit delayed in use at the start of combat (you can have it wind up while you're reloading it for later shots).

Re: Gear Rifles - design assistance requested
 

What TL are you thinking about? I was assuming this was for a clockpunk TL 5-6. The essence of 'clockwork' is that you have a primary energy source (for something portable, probably a mainspring, though non-portable clocks often used a suspended weight) that gets converted to something else through a set of gears, and if you can build a mainspring you've got pretty good elastic materials. Still, your basic problem is that most methods of converting rotary motion to linear motion involve long arms (the prototypical simple example is a torsion catapult).

Re: Gear Rifles - design assistance requested
 

For Oubliette, the world at large is mostly TL 3-ish, with some TL 4 technology - that is, your stereotypical DF setting. The dungeons are otherworldly in origin (and are their own pocket dimensions, after a fashion), and contain some technology from other worlds. Some of this technology, such as the gear rifles, is technically TL 3^ and TL 4^ clockwork, which performs roughly comparably to TL 5 and TL 6 clockwork. The inhabitants of Oubliette can duplicate this technology (in large part because one flavor of magic in the setting allows one to unmake an item, and in the process gain the knowledge to make it again, given appropriate time and materials... and skills), but as it is TL 3-4^, it is unreliant on the sorts of materials and technology available at TL 5-6, being able to use what they already have access to (iron, steel, etc).

What sort of material/technology did you have in mind? It may be something they could get access to via the dungeons, and potentially have other uses (for example, I'm considering the advanced knowledge of clockwork and gears allowing for decently-accurate timepieces and low-tech velocipedes, despite such not showing up as loot in the dungeon). I'll note I'm not aiming for scientific rigor, rather more "that makes sense, and would be pretty neat."

Re: Gear Rifles - design assistance requested
 

Good enough materials and manufacturing tolerances to build high strength low friction clockwork. Having orichalcum would probably help.

Re: Gear Rifles - design assistance requested
 

What about a series of spinning disks, probably in pairs, that propel a shot by friction? Each pair spins faster than the one before, to provide the acceleration along the length of the barrel? Somewhat like an automatic pitching machine in a batting cage, but in stages.

Each pair of disks has it's own clock spring. As the round moves down the barrel, it triggers each pair in turn. (This probably works better for a flechette or needle than a spherical shot.) The clock springs are all geared together, however, so that they can be wound from a master crank that is folded into the stock.

The basic mechanism isn't substantially more complicated than a wheel lock. The timing and master crank push this into the realm of unobtainium.

Re: Gear Rifles - design assistance requested
 

Well, at least the base models are unreliant on fantastical materials, just cinematic engineering. But are you saying something akin to my current setup, but with an intermediate clockwork storage system between the primary energy storage and the gears that actually move the chain?

Prior to seeing the video about the gauss rifle, this was more-or-less the working concept. However, the timing issue - as well as feeling hollow clay bullets may not be resilient enough to be battered about like this - made me dislike it. I also wasn't crazy about how heavy it would require the barrel to be - my preference is that roughly half the weight is in the stock (most of said weight being the energy storage mechanism), and half the weight being in the barrel, such that its balance point is around where the trigger is. I felt this would give it good balance for handling, as well as making it feasible to be used with Staff (it has comparable balance to a sword, where the balance point tends to be very near the crossguard, and I feel a sword should be usable with Staff, with one hand on the hilt and one on the blade).

Re: Gear Rifles - design assistance requested
 

Have you looked at GURPS Low-Tech itself? Page 79 has all sorts of devices for readying a crossbow or catapult to fire at a higher ST that the user's unaided level. See in particular the windlass and cranequin.

Re: Gear Rifles - design assistance requested
 

Actually, you can do the mechanical equivalent of a coilgun, though engineering it at TL 4, or even TL 6, is pretty optimistic.

The basic idea is that you have a pair of rails, with gears between the rails (probably more of a one-way gear, as it meshes more easily that way). The projectile is designed to slide along the rails, and has the ability to catch on the gears.

Now for the tricky engineering: each gear spins faster than the previous one, and thus as the projectile proceeds from one gear to the next, it is accelerated. Get the timing right, and you've got a mechanical linear accelerator.

Re: Gear Rifles - design assistance requested
 

Carefully chained nautilus gears might do the trick here, especially if you don't worry too much about strength of materials.

Re: Gear Rifles - design assistance requested
 

While I wasn't looking directly at them, I'll note that the windlass involves the same time multipliers as what falls out of my system, undoubtedly due to both being based on the same concept - that is, x1.5 to ST (and thus x2.25 to BL) is x2.25 to draw time, x2 to ST (x4 BL) is x4 to draw time, etc. So, I think my time ideas are on the right track. Arguably, it should use the doubled times of the cranequin, as it's only a single crank rather than the two of a windlass, but I'm willing to let that pass.

... of course, while looking up videos of windlasses to confirm that they have two cranks, I came across this gem. Yeah that's... that's a gear rifle. Shooting bolts, but a gear rifle (maybe the makers read the same thread I got the idea from? It was a gear bow there...). It's designed quite a bit differently from mine, however - but I think for DF, I kinda like my design better. But I'm biased.

Again, the idea isn't to have a mechanical coilgun, just a gun that uses mechanical energy instead of chemical. One of my other ideas was to have a piston (possibly telescoping, so it didn't stick out the back when not in use) that would engage gears much as you're envisioning the projectile doing, with the piston pushing the projectile forward. That would probably work a little better using the idea of a cup (basically, a small piston that gets carried forward by the sequence of gears), but I don't like the extreme precision that would be necessary to get the teeth of the gears and the slots on the piston/cup to line up as well as they'd need to, in addition to needing gears spinning at different speeds (for an accelerating effect), and the weight of having gears going all the way down the barrel (plus having each gear linked to the battery in the stock). What I like about the "chaingun" idea is that it's relatively simple - just gears in the stock spinning to spin a linked chain.

Re: Gear Rifles - design assistance requested
 

The problem with the chain idea is that you have to move the entire chain to move the projectile, which means you have a large efficiency loss unless either the chain is very light, or you can keep the chain spinning between projectiles.

Re: Gear Rifles - design assistance requested
 

Sounds as if you could be describing a scaled-up Red Ryder BB gun (HT, pp. 88-89). Skip the gears; use air as a working fluid. Put the piston under the barrel and back it with a strong coiled spring. Since you don't have to emulate a lever-action, you can increase the strength of the spring -- say, brace the butt on the ground and use two hands to cock it.

Not as delightfully clock-punk as a gear rifle, but certainly plausible. Real-world metallurgy and gun-smithing weren't up to the strengths and tolerances required, but you don't seem concerned about that.

Re: Gear Rifles - design assistance requested
 

We already have an Orichalcum Spring Gun in DF6. It's basically a ST12 Crossbow with 8 shots but without those inconvenient arms.

Re: Gear Rifles - design assistance requested
 

Hmmm... an excellent point. Two of the good reasons to use a chain (which I envision as a continuous loop on each side) are that the links themselves give something for the teeth of the gears to hook into, and that the cup can "slip" through some of them when it strikes the brake, so that the gears don't have to come to a sudden stop (which could damage them - and probably whatever chain/string is being used). For a string setup, which should markedly reduce the extra weight that needs to be accelerated, I feel we wouldn't want a continuous loop, but rather have one end of the string firmly attached to a spinning reel that would wind up the string. The "slip" would still be possible, by extending the string past the cup and tying knots in it (with a sufficiently large one at the end to make certain the cup doesn't slip off). However, once it's slipped past some of the knots, how would we get it back in front of them so it can slip next time?

Thinking on this, what if the tab (for pulling the cup back) isn't actually attached to cup, but rather to the ends of the strings? To pull it back into position, we have to pull the knotted portions of the string back through the cup. A large knot that it can't slip past, right in front of where the cup is meant to be, means when we pull the tab back, we pull the string back into proper position to allow slipping on impact, with the cup where it needs to be to be loaded. Does that sound feasible?

Containing highly-compressed air - particularly containing it for a decent chunk of time (these weapons are meant to be able to be loaded, then go at least several hours without firing, if needed, like firearms) - seems unfeasible to me when limited to ~TL3-4 materials. While I'm certainly not wanting strict scientific rigor, it makes more sense to me that superscience (well, cinematic anyway) methods could yield clock-punky mechanical batteries than that it can contain air compressed to the many atmospheres of pressure needed to have a weapon that is competitive with muskets. Probably just my own idiosyncrasies. Also, with regards to clockpunk, [tantrum]IwantitIwantitIwantitIwantit!!![/tantrum]

Sadly, I don't have that DF book. But it does sound like the same general concept, yes (but I don't want to be reliant on fantastical materials... particularly given an ~indestructible material like orichalcum may well not even exist in Oubliette, not even within the dungeons).

Re: Gear Rifles - design assistance requested
 

I wonder if a grosser movement than cranking could be a faster way to charge the weapon, like a long lever or a pull-cord.

Re: Gear Rifles - design assistance requested
 

Not meaning to quibble, but the point of the "Red Ryder" piston mechanism (vs. historical air guns, which did use a reservoir) is that the energy is stored in the spring, not in compressed air. You increase the pressure delivered by making the piston much larger than the bore. Another benefit is that the pressure is consistent, rather than diminishing as the reservoir empties.

Again, a spring (particularly a coiled spring) this strong isn't historically accurate, but it is probably minimally invasive as unobtainium goes. Maybe they have to be harvested from Hell Gnome traps...

Re: Gear Rifles - design assistance requested
 

I suspect not. A goat's foot takes 20 seconds to use and increases effective ST by 30-40%. A windlass takes 9 seconds to wind up the crossbow, and increases effective ST by 50%. Overall, I think using the two ends up taking a comparable amount of time, but that's because a windlass takes longer to attach and detach than a goat's foot. So I think winding is probably the most efficient. Note also that most devices that use muscle power to charge a battery also use cranks (some instead have you shake them, but personally I feel the winding ones work better).

Ah, I misunderstood you. Looking it up, it looks like you could base it on a fire piston, but with a narrow tube at the end the air can escape through... once it's accelerated the bullet wedged in there. It would still need to be something that you can charge over the course of several seconds (to generate enough force to propel the projectile at a higher effective ST), but seems workable, still with something soft to cushion the impact of the piston striking. It doesn't have all the clockpunk accoutrements, but has a certain simplicity that is enticing. And it makes rifling as simple as, well, just rifling the barrel. You do need the bullet to be largely flush with the walls of the barrel (to prevent air from escaping around it), but soft lead (or a softer wax jacket) should help with that. I'm not sure a flechette would be possible, although perhaps if it's contained within a sphere of wax? It seems like that would let it fit snugly in the barrel, then when it impacted its target the wax layer would be left behind, with the flechette simply cutting its way out and into the target. It also seems like a longer barrel wouldn't be as useful - for firearms a longer barrel means more of the powder burns before the bullet leaves, but this has all the pressure generated in an instant, going down as the bullet travels down the barrel. Perhaps this mechanism is (somehow) less efficient, but is used for pistols and short rifles, where its inability to benefit much from a longer barrel isn't an issue? I'll have to think on it.

Re: Gear Rifles - design assistance requested
 

Or just a flat plate plug as a sabot. Wax would add weight and drag and thus slow down the round.

Right -- but as long as the bullet is still plugging the barrel, it continues to get some residual acceleration from the pressure behind it even as that pressure drops. Ideally, I imagine you'd need the pressure to be above (1 atm + friction + drag at muzzle velocity) as the bullet exits the muzzle. That, in turn, should be a function of the ratio between volumes of air in the piston (before firing) and the barrel (after). Since both of these depend (in part) on the overall length, I suspect there's an optimal length for any given caliber.

Re: Gear Rifles - design assistance requested
 

Now that I've slept on it and am beyond my initial knee-jerk "But mine's better!" reaction, I'm really warming to the idea of an air piston.

For design, I'm thinking the piston and its chamber (which it never fully exits while in use) should be made of metal, with a soft leather stopper at the end of the piston, serving both to form an air-tight seal (which introduces more friction, but I feel that's necessary) and soften the impact when the piston reaches the end of the chamber. The piston, like the cup before it, would have a tab that comes out of the stock, but on the side rather than the top, for pulling it back into position after shooting. There would still be a safety lever, but this time it would be for being able to pull the piston back with ease (you disengage whatever pushes the piston forward - I'm thinking a pair of gears, with the piston having slots matching up to their teeth - from the battery, so it slides back without resistance, then re-engage it so you can shoot again; incidentally, I think a character with ST matching that of the weapon would be able to pull the piston back while it's still engaged, recharging the battery for one shot at the same time). I'm seeing reloading as a four second process - Ready to flip the safety and pull the piston back into position; Ready to draw a bullet; Ready to flip open the cover, drop in the bullet, and replace the cover; and a final Ready to reshoulder the weapon and flip the safety to prepare it to shoot again. Fast Draw reduces this to 3 seconds; the Rapid Reload Perk further reduces it to 2 seconds. This assumes your battery still has shots left, of course - otherwise you've got to draw and attach the crank, wind it up, detach and stow the crank... and then see the rest of the party is already looting the corpses.

I'm a little concerned about the air reservoir, but I feel I probably shouldn't be. When the character pulls the piston back, the only air supply to the reservoir is from the barrel itself, which has to pull the air from outside. For a character with Rapid Reload, we're probably looking at less than a second between the character starting to pull back the piston and blocking off the air supply with a bullet. Is that likely to be enough time, or should the chamber have air holes just in front of the piston when it's in ready position? Might that be a good idea anyway, in case the character goes out-of-order and loads the bullet before pulling back the piston?

I worry about the flechette maintaining its orientation in the barrel without something to stabilize it to the sabot, but feel when it exits and the sabot tears off (due to it having so much higher air resistance), whatever stabilized it is going to try to hold onto it, causing it to get deflected from what should be its path. Perhaps something like a flat plate, with thin rods going between the fins? That would hold up the flechette so it doesn't get out of proper orientation, but simply slide off when it hits the wall of air outside the barrel. If this would still be likely to distort flight (due to the plug not falling off evenly), you could have a detachable brake at the end of the barrel that catches the sabot, letting the flechette through; this would slow down the reloading time when using flechettes, as you'd need to pull off the brake and let the spent sabot fall out and put it back on, in addition to the other steps. This is probably doable with a two Readies (reduced to one with Fast-Draw) - tilt the weapon down, pull off the brake, let the sabot fall, then reattach the brake. How might this be sped up? Might it work to have a brake that can swing open, with springs to close it again and a lanyard going back to the base of the weapon? That seems like it would be one additional Ready (tilt the weapon down, pull back the lanyard while pulling back the piston, then release the lanyard for the brake to swing closed once the sabot falls out), or no extra time with Fast Draw. Is it feasible that something that could be swung open fairly readily would hold fast when the sabot struck it?

For the sabot itself, I think a soft wood would work alright. One possible concern would be heat, given we're basically looking at something akin to a weaponized fire piston, but I suspect (given the projectile is going to be accelerating even before the piston strikes the stop) the temperature won't get high enough to be an issue - at most, the back of the sabot might get a bit charred, but I'm thinking these things are single-use anyway.
EDIT: Gear rifles would have to be designed specifically to be able to shoot sabots, as you'd need a different shape of barrel cut-out to accommodate such, and probably something at the end for the brake to attach to. Such weapons would still be able to shoot normal bullets, of course, so long as the brake is removed before doing so (otherwise the bullet is likely to break the brake).

Every doubling of barrel length would halve the average amount of pressure during the acceleration. That makes a longer barrel a zero-sum game - doubling barrel length doubles the distance over which the force operates, but halves the average force (with constant area, pressure simply becomes force), and given kinetic energy is equal to force times distance, that gives us constant KE. Considering friction and drag, that actually indicates a longer barrel results in less velocity. Am I missing something? Firearms get to avoid this issue because their propellant continues to burn (offsetting the pressure loss of expanding to fill the barrel), but a gas-piston rifle has no such luck. As I mentioned above, the projectile should start accelerating before the piston strikes home, due to the constantly-increasing pressure, but once the piston has struck and we reach peak nominal pressure, any further barrel length is a net loss. It seems to me the bullet wouldn't travel very far before this happened... but perhaps I'm mistaken?

Or should I just flip physics another middle finger and say "I want longer barrels to result in more velocity, so that's what's gonna happen, and no more lip from you, Newton!"

EDIT: Just to get a glimpse, I decided to plug the damage for a "typical" gear rifle - 2d-1 pi, with a 10mm bullet - into DouglasCole's damage equation, to see how fast that bullet would be going as it would hit this velocity immediately following the piston striking home. The equation, for those playing at home, is Damage = (KE/cal^0.4)^0.5 * 0.6, with Damage in points (and corresponding to the average - 6 in this case), KE in Joules, and cal being the caliber in mm. That's 251 J (for about 63% efficiency, given it takes 4 seconds for an ST 10 person to wind, and said person can generally output around 100 W*). A lead sphere that is 10mm in diameter would mass 47.6 g, or 0.0476 kg. KE = 1/2 m*v^2, for a muzzle velocity of 103 m/s. Given the bullet starts at rest (velocity 0), that means the average velocity while it traveled down the barrel was 51.5 m/s. I wanted this rifle to have a 1 yard barrel; let's just call it 1 meter for simplicity. This indicates the piston took roughly 0.02 seconds to ram home.

Honestly? I think that works. So, no middle finger for physics, and sorry for calling you out, Newton, but I think this all works. I'll need to think on how this would impact optimal barrel length for other BL's - and projectiles... and then decide if that has the game effects I like, or if I should abandon all that. One thing this indicates is that clay bullets are going to see a damage reduction because rifles meant for firing lead bullets are too short for them (a 10mm clay sphere would mass somewhere around 5 g, for a muzzle velocity of 317 m/s, and average velocity of 158.5 m/s; with a 0.02 second power stroke, that calls for a 3.17 m barrel).

*This is sustained output, resulting in the character losing FP at something like 1 FP per hour (comparable to hiking). Someone trying to wind a gear rifle during battle is probably more akin to jogging than hiking, for doubled output (going All Out is akin to sprinting, for a further doubling of output). So, if we assume it would take 4 seconds for an ST 10 person using Readies in combat to wind up an ST 20 gear rifle, that indicates it's actually storing 800 J, for only around 30% efficiency.

Re: Gear Rifles - design assistance requested
 

Yeah. Pressure isn't constant in the barrel, so you have the same pressure at 6" away from the bullet entry point regardless of final length. Length typically doesn't give you really large velocity increases, but it's non-zero. In principle there are methods of introducing gas from the sides of the barrel that result in substantially higher velocities, but hard to do without electronic switches.

Re: Gear Rifles - design assistance requested
 

Ah, I think I see what I'm missing. For simplicity, let's assume our projectile doesn't start moving until we reach peak pressure. It is some distance, D, from the base of the barrel, and in the space between we're at peak pressure, generating a peak force, F. Because the cross sectional area of the barrel is constant, the force scales linearly with pressure. This starts accelerating the bullet. At distance 2D (total distance traveled of D), force has dropped to F/2, but average force between D and 2D would be equal to the average of F and F/2, or 0.75F, for kinetic energy equal to 0.75DF. At distance 4D (total distance traveled of 3D), force has dropped to F4, but average force between D and 4D would be equal to the average of F and F/4, or 0.625F, for kinetic energy equal to 1.875DF. It looks like you rapidly reach a steady state where each additional D worth of length adds +0.5DF to kinetic energy (technically, +0.5DF per +D is the limit), so in terms of the % increase this keeps dropping, but it is indeed a net gain indefinitely (although eventually you're going to reach the point where losses from friction and drag cancel it out).

Looking at our example rifle - 2d-1 pi, 10 mm, 1 meter long barrel - and assuming the projectile is located 10 mm from the piston chamber (so D is equal to projectile diameter), we're at 100D, and our kinetic energy is at 49.995DF. Setting this as exactly 6 damage (2d-1), and rounding normally (so 6.5 is 7), we need to go to 1.18m for +1 damage, 1.57m for +2 damage, and 2.01m for +3 damage. Meanwhile, shortening our weapon to 0.85m has no effect on damage, going as short as 0.57m is -1, 0.35m is -2, and 0.18m is -3. If instead round down (so 6.999 is still 6), we need to go to 1.37m for +1, 1.78m for +2, and 2.25m for +3; meanwhile, -1 to damage can be as short as 0.7m, -2 can be 0.45m, and -3 can be 0.26m.

I'll play around with the values and see what falls out. Thanks!

EDIT:
Me: "I'm not aiming for scientific rigor."
Also me: "Let's integrate the pressure curve to see how KE and damage change with the length of the barrel!"

Re: Gear Rifles - design assistance requested
 

Took a bit to get back into messing with this, with how much of a downer Real Life has been recently (if you don't live under a rock, you probably know what I'm referring to; if you're currently literally living under rock because some a**hole looked at your country and cried "IwantitIwantitIwantit," you definitely know what I'm referring to), but at least for now, back to messing around with it. Things are coming together, but I'm a bit stuck where I always was with the firearm design - MinST. This seems like it would be the greater of whatever ST is needed to hold the weapon in place (I'm currently thinking one-handed firearms can be up to BL/2, while two-handed ones can be up to BL), and whatever ST is needed to manage recoil. That latter is the sticking point. I've read that some old British guidelines suggested no more than 20 lb-force of recoil for a service rifle, and that most modern service rifles are around 15 lb-force of recoil, so I'm tentatively saying something like the above - BL/2 for one-handed firearms, BL for two-handed ones. The issue is... I don't know how to calculate this for my gear rifles. I can work out the recoil energy for them, provided I ignore the secondary recoil from accelerating the working fluid (I think this can be done safely in this case). After messing with the math a bit*, it looks like the recoil energy is equal to the muzzle energy multiplied by the mass of the bullet, divided by the mass of the rifle - Rk = Bk*Bm/Rm. But that gives me energy, and the values I have to work with to set "Is this wieldable?" are in terms of force. Now, given that kinetic energy is equal to force times distance, I can get force by dividing by distance... but what distance is at play here? The length of the barrel? How far the rifle moves back while dumping its energy into the shooter's hands/shoulder? I'm thinking it's the latter but... what would that actually be, assuming a competent shooter with minimal or no padding? How much would it change if the shooter is wearing a gambeson or similar padding?

*For those playing along at home (and because I'd like someone to check my math to make certain I didn't screw something up), the variables of note are Bm (mass of the bullet), Bv (velocity of the bullet), Bk (muzzle energy), Rm (mass of the rifle), Rv (velocity of the rifle), and Rk (recoil energy). We start with conservation of momentum, which gives us
Bm*Bv = Rm*Rv (there should be a negative sign here, given velocity is a vector, but I only care about absolute values, so we ignore that)
We need kinetic energy in the equation, so we use
Bk = 0.5*Bm*Bv^2
Rk = 0.5*Rm*Rv^2
I opted to solve this for Bm and Rm, so that I could avoid needing to bother with square roots
Bm = 2*Bk/(Bv^2)
Rm = 2*Rk/(Rv^2)
Substituting these into the momentum equation (Bm*Bv = Rm*Rv), we get
2*Bk*Bv/(Bv^2) = 2*Rk*Rv/(Rv^2)
Which simplifies down to
Bk/Bv = Rk/Rv
And solving for Rk gives us
Rk = Rv*Bk/Bv
Of course, we don't know what velocity our rifle is getting accelerated to, and frankly we don't care. Fortunately, we can solve our momentum equation for Rv, giving us
Rv = Bm*Bv/Rm
Substituting this in with our recoil energy equation, we get
Rk = (Bm*Bv/Rm)*Bk/Bv
Which simplifies down to
Rk = Bk*Bm/Rm

Re: Gear Rifles - design assistance requested
 

If it's expressed in units of force it's almost certainly in error, the peak instantaneous force for a .223 rifle is in excess of 2,000 lb and that's a relatively low recoil rifle. I suspect the sources you were looking at were actually giving foot-pounds, which is a unit of energy.

Re: Gear Rifles - design assistance requested
 

I think I may have jumbled some of the sources I was checking together; the less-reliable ones talked about pounds, but the one I was remembering with the note of most service rifles being around 15 was indeed talking about foot-pounds, which simplifies matters. Rereading that one, I'm considering that perhaps the maximum recoil before you start suffering penalties* is more around 3/4*BL in ft-lb for a rifle (so 15 ft-lb for a character with ST 10 and thus BL 20). I'm fairly confident this is going to mean the weight of the weapon will be the limiting factor for most gear rifles - I think I worked out my reference weapon would be roughly 5 lbs (assuming twice the weight of a comparable firearm), so 251 J - 185 ft-lb - in a 0.1 lb bullet corresponds to a mere 3.7 ft-lb recoil for a 5 lb rifle.
(EDIT: Actually, now that I look at it, if I use BL for weight and 3/4*BL for recoil, those two call for the same ST - ST 5 gives BL 5, and 3/4 of that is 3.75)

It seems like the gear rifle would have an odd recoil profile, although I could be mistaken, but I don't think this would really have a detrimental effect (should just be part of Familiarity). Basically, you get traditional rearward recoil from the piston being pushed forward and the bullet being propelled, but you'd also have some forward recoil when the piston gets arrested upon striking. For most rifles, I think the piston slamming into its base is probably the noisiest part of "firing" the weapon - although my spreadsheets do indicate that pi- (5 mm) rifles break the sound barrier at around ST 17, pi (10 mm) rifles do so around ST 27, pi+ (15 mm) rifles do so around ST 37, and pi++ (20 mm) rifles do so around ST 47. This would technically require the compressed air to get rather hot, so that the speed of sound in the air increases... but this is a modified fire piston, so I'm alright with that, even if it isn't strictly realistic.

*Realistically, above a certain threshold, I think it's more that you can manage some number of shots before the soreness starts damaging your accuracy, but for GURPS purposes, a set cut-off works best.

Re: Gear Rifles - design assistance requested
 

One option I forgot to list in the gauss rifle thread that seems steampunky would be the method famously used in the Whitworth rifles (both small arms and cannon) in the late 19th century (American Civil War era, though Whitworth was English). Hexagonal barrel with a hexagonal projectile to match. (The small arms had hexagonal cross sections; the cannons had projectiles that were also twisted, so they look spiral-shaped.) The orientation of the hexagon rotates down the barrel. So, no grooved rifling that bites into the bullet; instead, the has flat surfaces that match the barrel all the way around, rotating as it moves down the barrel because the barrel itself twists. Your plan started with an elongated bore to have room for the chains; rotate that all the way down the barrel.

The Whitworth small rifles would also fire cylindrical bullets if they were made of soft lead. Polygonal rifling without the fancy ammo has been used in a lot of weapons, including fairly modern ones.

Anecdotes say that the Whitworth cannon projectiles made a distinctive and peculiar screech as they flew through the air, which also might be an appealing detail just for color.

Re: Gear Rifles - design assistance requested
 

ACW-era is a bit late for what I have in mind, but those do sound pretty awesome. I was considering smoothbore longarms (powder or piston driven) to have Acc 2 (pistols would have Acc 1), while rifled ones would have Acc 3 (pistols would have Acc 2), with double barrel cost (I separate the cost of the barrel, the breach, and the stock for firearms; for gear rifles, the piston mechanism and battery would also have separate costs). The Wikipedia entry on the Whitworth notes it had increased accuracy but was around 4x the cost of a Lee-Enfield. I'm thinking such hexagonal barrels may well be available; they'd increase Acc by a further +2 for longarms, or +1 for pistols, with the option in either case to get another +1 by using special expanding ammunition (like the soft lead bullets of the Whitworth, which had a Minie-style expansion when fired); barrel cost would be x10 compared to smoothbore (x5 compared to rifled), and the special ammunition would be +2 CF. This is in addition to the options to make the weapon Fine (Accurate) for +0.75 CF and +1 Acc, or make the ammunition Balanced for +4 CF and +1 Acc (a dedicated marksman's rifle would have a hexagonal barrel, be Fine (Accurate), and use Minie-style, Balanced bullets, for a grand total of Acc 8 - on par with a modern sniper rifle firing match-grade ammo, which is pretty ridiculous but... whatever, it works).

Re: Gear Rifles - design assistance requested
 

The comparison is with an Enfield, formally a Pattern 1853 Enfield. The Lee-Enfield didn't come along until 1895.

Re: Gear Rifles - design assistance requested
 

Ah, thanks. It seemed too early for the Lee-Enfield, but I thought maybe it was just that there were a lot of rifles that were given that name; guess my mind just threw the "Lee-" onto it.

Re: Gear Rifles - design assistance requested
 

I used something similar, specifically orichalcum spring gun, in Desolation Road. Rather than rifling I assumed finned bolts.

I also had advanced Wheelock revolvers called "clocklocks" and resonating crystal alchemical "blasting rods".

Re: Gear Rifles - design assistance requested
 

My current inclination, with regards to MinST, is to say that rifles and the like use BL/2 (so a 15 ft-lb rifle actually calls for someone with BL 30, or around ST 12... which is honestly probably appropriate for modern infantry), while pistols and the like use BL/4; using a pistol two-handed, or firing a rifle without the stock, would probably use BL/3, while firing a rifle with a bipod or similar would use BL. That's for sustained use - you can fire a much heavier weapon, up to twice the above (BL, BL/2, 2/3*BL, and 2*BL, respectively), but you must pay 1 FP per shot to do so. Does this sound feasible? Again, you'd use the higher MinST between weapon weight (up to BL lb for rifles, up to BL/2 lb for pistols; weapons using bipods and the like probably 2*BL) and recoil energy. I feel the former would use Lifting ST, while the latter would use Striking ST, but I could be mistaken (if the character has different Lifting ST and Striking ST, his/her Lifting ST must meet the weight requirement, while his/her Striking ST must meet the recoil energy requirement).

Re: Gear Rifles - design assistance requested
 

A top-mounted magazine need not interfere with the sights - both the Bren Gun and the Owen Submachinegun had top-mounted magazines and sights (the sights are off-set to one side). What it does do is stop you aiming the weapon 'wrong handed'.