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Old 03-09-2014, 02:54 PM   #11
lwcamp
 
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Originally Posted by BraselC5048 View Post
Well, I certainly didn't realize recoil was based on momentum, not muzzle energy
Physics says that momentum going one way is ballanced by momentum going the other - hence the gun recoil. However, recoil energy seems to correlate better with what people talk about when complaining about a gun's recoil, or discussing its ability to throw off your aim. This is the kinetic energy of the gun as it recoils. Find the recoil velocity by dividing the recoil momentum by the gun's mass (this is from the usual formula for momentum, P = MV). Then square that velocity, multiply by the gun's mass, and divide by 2 (the usual formula for kinetic energy, E = M V^2/2, which is equivalent to E = P^2/(2 M)).

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Old 03-09-2014, 03:16 PM   #12
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Originally Posted by lwcamp View Post
Physics says that momentum going one way is ballanced by momentum going the other - hence the gun recoil. However, recoil energy seems to correlate better with what people talk about when complaining about a gun's recoil, or discussing its ability to throw off your aim. This is the kinetic energy of the gun as it recoils. Find the recoil velocity by dividing the recoil momentum by the gun's mass (this is from the usual formula for momentum, P = MV). Then square that velocity, multiply by the gun's mass, and divide by 2 (the usual formula for kinetic energy, E = M V^2/2, which is equivalent to E = P^2/(2 M)).
That kind of makes sense. But then

Bullet mass scales as R^2 [R^3]
Bullet velocity scales as 1/R [1/R^1.5], assuming constant kinetic energy
Bullet momentum scales as R [R^1.5]
Gun momentum also scales as R [R^1.5]
Gun velocity also scales as R [R^1.5], assuming constant gun mass
Gun kinetic energy scales as R^2 [R^3]

Or, in other words, if you shoot a big, slow bullet, more of the kinetic energy seems to go into pushing the gun back at you.

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Old 03-09-2014, 03:21 PM   #13
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Originally Posted by lwcamp View Post
However, recoil energy seems to correlate better with what people talk about when complaining about a gun's recoil, or discussing its ability to throw off your aim.
Arguably these aren't the same thing and should be reflected in different ways in the gun's stats. The amount by which recoil throws a gun's aim off is proportional to (recoil velocity) * (time required to correct for that velocity). Correction time for muscles will be something like (muscle response time + recoil momentum / muscle force). Since ST doesn't reduce the GURPS recoil stat, presumably Rcl is supposed to mostly be a function of response time, and should be based on (recoil momentum)/(gun mass), while the gun's ST score should be dependent on recoil energy.
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Old 03-09-2014, 03:44 PM   #14
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Originally Posted by whswhs View Post
Or, in other words, if you shoot a big, slow bullet, more of the kinetic energy seems to go into pushing the gun back at you.
That's generally how these things seem to work out - lighter bullets mean an easier to control firearm.

Also, lighter bullets mean a flatter trajectory, greater proportional energy loss at a given range, and less penetration.

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Old 03-09-2014, 04:13 PM   #15
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

Shouldn't acceleration of guns matter just as much, if not more, than overall amount of energy imparted?
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Old 03-09-2014, 04:18 PM   #16
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Shouldn't acceleration of guns matter just as much, if not more, than overall amount of energy imparted?
Seems unlikely. The firing of a single shot occurs in such a short duration that treating it as an instantaneous impulse won't be too far off.
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Old 03-09-2014, 04:33 PM   #17
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

Recoil doesn't really matter all that much in a revolver, since the one time my character pulled it out, it was "pull the trigger once each round," then see if the target needed another one. It really matters more as to whether or not it's actually a practical combat weapon. (The navy has decided that revolvers really are the best choice since they don't get a 10% malfunction rate in a vacuum, which is most of the time. If you're going to have a backup gun, you might as well ensure it's actually going to work.)

So, how much should this revolver weigh? 3.1 lb or 3.2 lb? And can I justify recoil 3 - somewhat high, but manageable, and not so high it makes it impractical (see: .44 magnum)?

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Old 03-09-2014, 04:43 PM   #18
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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(accuracy again - is there a rule about aiming bonuses past 1/2d in 4e?)
There is not. Aiming works the same at all ranges, and 1/2d is totally irrelevant to the attack roll.

Tactical Shooting offers a penalty for shots with flight time over a second, though. (I hadn't previously thought about it but it's at least partially specific to shooting in an open atmosphere.)
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Originally Posted by BraselC5048 View Post
Recoil doesn't really matter all that much in a revolver, since the one time my character pulled it out, it was "pull the trigger once each round," then see if the target needed another one. It really matters more as to whether or not it's actually a practical combat weapon. (The navy has decided that revolvers really are the best choice since they don't get a 10% malfunction rate in a vacuum, which is most of the time. If you're going to have a backup gun, you might as well ensure it's actually going to work.)

So, how much should this revolver weigh? 3.1 lb or 3.2 lb? And can I justify recoil 3 - somewhat high, but manageable, and not so high it makes it impractical (see: .44 magnum)?
Why is high rcl making the gun impractical if you normally fire one shot per second?
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Old 03-09-2014, 05:56 PM   #19
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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There is not. Aiming works the same at all ranges, and 1/2d is totally irrelevant to the attack roll.
Ok, thanks, just happened to see it when flipping through (3e) GURPS:WWII, and was wondering if it was carried over into 4e.


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Tactical Shooting offers a penalty for shots with flight time over a second, though. (I hadn't previously thought about it but it's at least partially specific to shooting in an open atmosphere.)
Somehow I never noticed that in my copy. I'll give it a read. I've also noticed that combat ranges in boarding actions are always less then 200 meters (even the longest battleship in service with any navy is around that length), and usually from 20-30 meters at most, down to point blank. The main effect of vacuum would be that 1/2d range is unlimited, although you'll be stopped by a bulkhead long before that. There is superscience artificial gravity, after all.

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Why is high rcl making the gun impractical if you normally fire one shot per second?
In game terms, it really doesn't. The issue is more of an in-universe thing. Nobody would ever use a .44 magnum as a combat service revolver - it's just too powerful, and too impractical. A .357 magnum is about as powerful as you would actually want to use in combat, or as an issue weapon. Go too much more powerful then that, and the extra power isn't worth the tradeoffs. A .357 magnum is considered a viable caliber for police work, and pretty much the most powerful one, while no one considers the .44 magnum to even be worth thinking about.
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Old 03-09-2014, 06:07 PM   #20
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Default Re: Gun design: Muzzle energy, bullet caliber, and recoil

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Originally Posted by BraselC5048 View Post
Somehow I never noticed that in my copy. I'll give it a read. I've also noticed that combat ranges in boarding actions are always less then 200 meters (even the longest battleship in service with any navy is around that length), and usually from 20-30 meters at most, down to point blank. The main effect of vacuum would be that 1/2d range is unlimited, although you'll be stopped by a bulkhead long before that. There is superscience artificial gravity, after all.
Bullet travel is in the Harsh Realism box on page 32.

Boarding any sort of normal vessel, yeah, you wouldn't see bullet travel being a problem for firearms. I wasn't sure, given the vacuum, whether you were going to be doing shooting outside, where ranges can get (arbitrarily) long.
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Originally Posted by BraselC5048 View Post
In game terms, it really doesn't. The issue is more of an in-universe thing. Nobody would ever use a .44 magnum as a combat service revolver - it's just too powerful, and too impractical. A .357 magnum is about as powerful as you would actually want to use in combat, or as an issue weapon. Go too much more powerful then that, and the extra power isn't worth the tradeoffs. A .357 magnum is considered a viable caliber for police work, and pretty much the most powerful one, while no one considers the .44 magnum to even be worth thinking about.
Even .357 magnum seems excessive, really! Unless the service doesn't agree with the consensus that a pistol is the weapon you use when you can't get at a proper (shoulder) arm. But I'd think it less about anything covered by rcl than with ST (you probably don't want an issue sidearm above ST 9), weight and bulk (convenient portability is invaluable), and cost per shot (you have to provide ammo for training).
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