Bullet flight time conventions

[Joseph Paul]

Originally Posted by Fred Brackin

What rule would this be? Can you cite a page number? For the 64mm and 100mm missiles the two distances shown are indeed velocity per Turn and max range but I do not believe anything is said about what happens when you put guidane on conventional bullets. The passage about doing that could be expunded with little loss.

To the OP I can tell you that the velocity increase for ETC is 50%, same as for damage. Otherwise the velocities for plain caseless rounds should proably be not very different from those of contemporary guns.

Finally, What do _I_ do about this in my games? Nothing. It's never come up.

The expounding was done on UT p 146. There it explicitly allows the use of homing projectiles fired from guns. All homing projectiles use the rules on Basic 413 which calls for the round to use 1/2D as speed per second. If the round is homing it appears that the firer needs Artillery(Missile) as the skill to launch it even if it is fired from a conventional gun.

It doesn't make sense and I am trying to get all of those rules across GURPS together to hash out what I want to do.

Given what you have said elsewhere that your group has little interest in military action I am not surprised that you haven't had to deal with it.

I am building scenarios where PCs will be shot at from beyond 1/2D or TS's 600 yard range and notice the thermal bloom in their hyperspectral goggles and must decide on racing that round to the nearest cover or grabbing the bad guy as a human shield... So what would you do about that?

[Joseph Paul]

Originally Posted by Rupert

<SNIP>

For UT weapons any 1/2D is going to be an approximation, and max range an approximation off that approximation. There's no sensible way around this.

I am thinking I want different approximations.
A lot of your post will be getting a closer examination in that regard so thank you for it.

[DouglasCole]

Originally Posted by Rupert

If 1/2D range is looked at as being when the projectile has slowed to the point where it does half the damage it does at the muzzle, then many of the ranges will be wrong. 1/2 damage implies 1/4 velocity, which can be a surprisingly long way.

Nitpick: Half velocity.

KE = 1/2 M V^2, and damage is based on the sqrt(KE). So damage is linear in velocity. (linear-ish)

[Fred Brackin]

Originally Posted by Joseph Paul

I am building scenarios where PCs will be shot at from beyond 1/2D or TS's 600 yard range and notice the thermal bloom in their hyperspectral goggles and must decide on racing that round to the nearest cover or grabbing the bad guy as a human shield... So what would you do about that?

If they can percieve a threat they may use an Active Defense agisnt it but other actions require that they get their full Turn. That's more like 2 seconds flight time than 1. Still if they have their full Turn before the projectile reaches them they can take any legal Action in that Turn.

If they do move to another hex I'd probably just declare an auto-miss. Very long range sniping requires the target to be unaware of the coming attack.

However, do check the Vision mods for noticing the firing. When the Mythbusters messed about with "dodging" bullets they found no realitic way the target could spot the firing at any distance (i.e.hundreds of yards) where he'd have time enough to act. 1000 yards is -16 to Vision rolls as well as Guns Rolls. Even if you give the firing the -10 for "in plain sight" s a suppressor (or maybe just a"dry" one there's text about "wet" suppressors reducing IR flash too) doesn't hide the flash from the target
there's still another -6 to make up.

I have done some rifle combat in a Cliffhangers game but that resolved itself as "Aim" and "Fire" on alternating Turns at about 100 yards. Nobody would have hit anything at 1000 yards even if they had spotted anyone that far away.

If I did want to make PCs wary about long range sniper fire I'd proceed dirsectly to "the "crack!" sound and a bullet hitting a rock a few yards away. Force a PC to spot a firing rifle a long way away and you could end up killing him for failing a Per Roll.

[Ulzgoroth]

Originally Posted by Joseph Paul

I am building scenarios where PCs will be shot at from beyond 1/2D or TS's 600 yard range and notice the thermal bloom in their hyperspectral goggles and must decide on racing that round to the nearest cover or grabbing the bad guy as a human shield... So what would you do about that?

So smart bullets aside, what are you doing with dodging dumb projectiles? Against single shots that you can see being fired more than a second out, I'd think a target willing and able to move would be almost incapable of failing to dodge. Even an automatic spread would be mostly hopeless.

[Gigermann]

Originally Posted by Ulzgoroth

So smart bullets aside, what are you doing with dodging dumb projectiles? Against single shots that you can see being fired more than a second out, I'd think a target willing and able to move would be almost incapable of failing to dodge. Even an automatic spread would be mostly hopeless.

If it takes >1 sec to arrive, and you see it, you don't have to Dodge—you just move to another hex :P

[Anthony]

My rule of thumb is that for anything with a long bullet, muzzle velocity is 1,000 yps * (damage in dice) / (bore size in mm).

As that's a bit low for pistols (a 9mm at 2d+2 works out to 2500/9 = 278 yards per second), mostly because lower power bullets are often also short compared to their diameter, using around the 0.75 power of (damage/bore size) might give better numbers.

As for dodging dumb projectiles: realistically, if you see the shot in time to react and move, the only way it can possibly hit is if the attacker missed and randomly hit where you chose to dodge to.

[Varyon]

Originally Posted by Joseph Paul

I see the possibility of defining muzzle velocities for some of these weapons and playing with variations. I am wondering if there is a muzzle velocity advantage that I can give to EM firearms that will keep them ahead of conventional firearms and even with ETC/ETK?

This is far from perfect, but you can actually extrapolate muzzle velocities from the weapons presented in Spaceships, as damage there scales linearly with both caliber and velocity.

A 40mm Spaceships projectile traveling at Move 1760 (1 mile per second) does 60d damage at normal scale. That rounds up to 1d per 30 yards/second. Dividing that by 40 gives 1d per mm per 1200 yards/second. So, if you divide damage by caliber in mm and multiply the result by 1200, you'll get the round's velocity in yards/second.

For the Payload Rifle (UT137), for 10d out of a 25mm projectile you need Move (1200*10/25)=480. A Boosted LP shot does 10d+10, or around 13d - Move 624. A low velocity LP shot does 5d - Move 240, which is indeed subsonic. Personally, I prefer placing "subsonic" at around Move 350, which would actually do around 7.3d - 7d+1 - damage, rather than only 5d. Each shot uses up around 0.5 shots of propellant.

For the Gauss Rifle (UT142), for 6d+2 (6.7d) out of a 4mm projectile you need Move (1200*6.6/4)=1980 (over 1 mps). A boosted shot does 6d+8 (8.3d) - Move 2490. A low velocity shot does 3d+1 (3.3d) - Move 990. Note this is still a supersonic shot; throwing the projectile at Move 350 would only do 1d+1 damage, but would only use up around 3% of the power of a typical shot.

An optional additional step (well, several steps really) would be to compare the WPS of your rounds to those of Spaceships rounds. This is complicated by the fact Spaceships rounds things a bit and occasionally nudges things to be more in line with SSR, which throws things off a bit. Here we'll deviate at bit from Spaceships since we need to be able to work with arbitrary calibers without worrying how well they fit into the table. First off, Spaceships conventional guns tend to have bullets weighing twice those of comparable Gauss/grav guns, so we can assume 50% of the mass is in propellant. Going off of the 1 ton (2000 lb) weight of a 40 cm Gauss/grav bullet, and scaling by the cube of caliber (which Spaceships does prior to the rounding issues coming into play) gives us a 1 lb bullet at around 32mm. Such a bullet would need an equal weight of propellant to reach Move 1760. For a different Move, we know from LP that the weight of propellant required scales with the square of velocity. For bullets of weights different from what was predicted above, from the way collisions work we can assume damage scales with the cube root of weight (and indeed scaling linearly with caliber supports this).

Given this relationship, if we run into a discrepancy with a conventional round, increasing bullet weight (with corresponding decrease to propellant weight) will overall increase the weight of the round. Increasing propellant weight (with corresponding decrease to bullet weight) will overall decrease the weight of the round. Every xn to bullet weight (x(n^1/3) damage) needs x(n^2/3) to propellant weight (x1/(n^1/3) damage) to balance it out. Unfortunately, I don't think there's an equivalent to the quadratic equation that will help us out here, but I'd be happy to be proven wrong. The equation will look something like an^(2/3)+bn=w, where a is the SS-calculated weight of propellant (accelerant), b is the SS-calculated weight of the bullet, and w is the weight of the round in UT. From here, divide mass of propellant by mass of the bullet, take the square root, and multiply by 1760 to get muzzle velocity. Note this assumes comparable propellants between UT and SS, which isn't necessarily the case.

For Gauss/grav, it's far easier - simply divide calculated weight by UT weight, take the cube root, and multiply by 1760 to get muzzle velocity.

Our 25mm bullet above should thus weigh around ((25/32)^3) 0.48 lb. 0.48 additional lb of propellant would boost it to Move 1760; with it only having a Move of 480, it needs around ((480/1760)^2) 7.5% as much propellant, or about 0.035 lb. for a total weight of 0.515 lb. The 25mm rounds actually weigh 1 lb, so we need a heavier bullet. 0.035(n^2/3)+0.48n=1 is our equation; messing around in a spreadsheet, I get around n=1.968. That gives us a bullet weight of 0.945 lb and a propellant weight of 0.055 lb. With propellant being 5.8% of the mass of the bullet, this gives us Move (1760*0.058^1/2)=425 or so. This really isn't much a difference from our above calculation of Move 480.

Our 4mm Gauss bullet above should thus weigh around ((4/32)^3) 0.002 lb. UT gives a weight of 0.006 lb. This gives us Move (((0.002/0.006)^1/3)*1760) 1220. That's a notable difference from the 1980 calculated above; a low velocity, Move 350 projectile is now around 30% velocity and damage (instead of something like 15%), for 2d pi- (instead of 1d+1 pi-).

Originally Posted by Fred Brackin

If they do move to another hex I'd probably just declare an auto-miss. Very long range sniping requires the target to be unaware of the coming attack.

It's not like the victim is aware of exactly where the sniper is shooting. If the sniper accurately predicts where the victim will go in response to noticing the shot (which would require supernatural or at least cinematic abilities on the part of the sniper, but GURPS allows for those), a hit is still entirely possible. As you note, of course, the safe money is on the victim being unaware - at the distance the characters would need to be apart for a supersonic* projectile to leave enough time for the latter character to actually move a yard or more, noticing the flash (and responding immediately) is rather unlikely.

*For a subsonic shot, a failed Vision roll to notice the muzzle flash or whatever may allow for a subsequent Hearing roll to hear the shot, as the sound reaches the character before the bullet does. Of course, as the speed of sound is roughly 375 yards per second, even a slow 250 yard/sec gives little time to respond - at 500 yards (an impressive shot for such a slow bullet) the sound reaches the target only 2/3rds of a second before the bullet.

[Rupert]

Originally Posted by DouglasCole

Nitpick: Half velocity.

KE = 1/2 M V^2, and damage is based on the sqrt(KE). So damage is linear in velocity. (linear-ish)

Gah. I knew that, really.

[Joseph Paul]

Originally Posted by Ulzgoroth

So smart bullets aside, what are you doing with dodging dumb projectiles? Against single shots that you can see being fired more than a second out, I'd think a target willing and able to move would be almost incapable of failing to dodge. Even an automatic spread would be mostly hopeless.

I think that one of the things my setting is going to test is that with integrated detection capability participants in hot conflicts are going to find that long range is not a help. HUD displays linked into hyperspectral sensors on drones and radscanners (as a catch all for a future integrated EM&RAD sensor suite) will catch the firing of conventional guns and EM guns. Which leaves lasers as the choice of snipers 'cause you can't beat c.

When the sniper alarm goes off people move and try for cover. If everyone just hits the dirt snipers will aim low in front of their target to try to generate hits. So targets should try to be a little more erratic in their evasion. So a homing bullet might work because it would be able to make some corrections. Maybe. I have little confidence that small caliber rounds would be able to make much of a course change given they have no thrust to work with and often little altitude to trade. The vision of them doggedly pursuing their mark through an urban apocalypse dodging buildings and the odd burnt out car is not one that I cater to. Whether they would actually help with AA fire or air-to-air combat is something to be considered.

But the original question was to find out what the spread of options were in the RAW GURPS at this point. To clear up the clutter of rules I think I am going to set some specific muzzle velocities for UT to work with long range shots and work out my own weapons tech progression. It appears that I will need to make some setting decisions about what can have Guided/Homing war heads and what can't and be very explicit in the how-things-work department.