Gear Rifles - design assistance requested

[Varyon]

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?

Originally Posted by thrash

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

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).

Originally Posted by thrash

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.

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.