Gun design: Muzzle energy, bullet caliber, and recoil

[BraselC5048]

Anyway, I've been stating up some fictional guns. The thing is, can you increase the caliber without increasing the muzzle energy (not a big a loss of velocity, since it is squared in the equation, after all)? And keep the recoil of the smaller caliber?

Two specific guns:

A .455 revolver, with the muzzle energy and recoil of a .357 magnum. So 3d damage, recoil 3, although firing a heavier bullet - is that right?

A .455 SMG (the revolver takes it's ammunition, but not vice versa), based on the KP/31 Suomi, much more out of GURPS:WWII then High Tech, with appearance crossed with a Lanchester Mk I. Muzzle energy is the same as a 9mm, 3d-1. Acc 4, despite the lower energy, keeping the Suomi's reputation for accuracy, even if a .455 SMG should get Acc 4 anyway. 1/2d is 200 yards (accuracy again - is there a rule about aiming bonuses past 1/2d in 4e?), as well as the Fine (reliable) that it should have had in High Tech to begin with. Recoil is 2, from having the muzzle energy of a Recoil 2 9mm SMG. Does that all sound about right?

[whswhs]

Originally Posted by BraselC5048

Anyway, I've been stating up some fictional guns. The thing is, can you increase the caliber without increasing the muzzle energy (not a big a loss of velocity, since it is squared in the equation, after all)? And keep the recoil of the smaller caliber?

I'm not an expert on firearms engineering, but I can look at the basic scaling laws from dimensional analysis.

Let's say the original caliber was x, and the scaled caliber is Rx, where R is the ratio; that is, we increase caliber by a factor of R. Bullet mass then increases by a factor between R^2 (if you make the bullet thicker but no longer) and R^3 (if you scale up the length in exact proportion). Since muzzle energy is unchanged, and since KE = 0.5mv^2, v^2 = 2KE/m. So v decreases by a factor of between R and R^1.5.

Now, recoil is the momentum of the gun, which is necessarily equal to the momentum of the bullet. Momentum is mass times velocity. Mass increases by between R^2 and R^3; velocity decreases by between R and R^1.5; that leaves momentum increasing by between R and R^1.5. That is, the base impact necessarily gets bigger.

However, it may well be delivered less sharply. The gun presumably isn't any longer (same muzzle energy!). But its barrel is going to have a greater circumference, and thus is going to weigh a bit more. Greater weight of the gun will mean that the velocity with which the gun hits the shooter's hand or shoulder will not increase so much—conceivably not at all, but I suspect that's an extreme case. Mostly like I think is that the recoil velocity goes up, but modestly.

I may be overlooking some design feature. But it looks to me as if the recoil ought to be at least marginally greater.

Bill Stoddard

[Dalillama]

Originally Posted by BraselC5048

Anyway, I've been stating up some fictional guns. The thing is, can you increase the caliber without increasing the muzzle energy (not a big a loss of velocity, since it is squared in the equation, after all)? And keep the recoil of the smaller caliber?

AFAIK the muzzle velocity usually doesn't go up with the caliber. If anything it's the opposite.

[Fred Brackin]

Originally Posted by BraselC5048

Anyway, I've been stating up some fictional guns. The thing is, can you increase the caliber without increasing the muzzle energy (not a big a loss of velocity, since it is squared in the equation, after all)? And keep the recoil of the smaller caliber?

First thing is that "muzzle energy" is probably not a useful term in dealing with Gurps firearms. It certainly doesn't help me anyway.

Second, it will work much better to take a truly similar firearm and compare that to your proposed fictional gun. A .357 does not qualify at all. You need something with a bullet of similar weight

So a .455 revolver is indistinguishable in Gurps terms as to caliber from .45, .450, .454 or several others. Now a .455 that does a full 3D is a good bit hotter than a .45 (actually .454) Colt at an average of 8.5pts of damage but not nearly so hot as a .454 Casull at 16.5.

The .45 Colt from the 3 lb SAA gets recoil 4 while the .454 Casull gets a 5 from the Super Redhawk at 3.6lbs. So it's either 4 or 5 depending on where the breakpoint is but it's definitely not 3.

A .455 SMG that does 3D-1 (9.5pts) is definitely higher velocity than a .45 Thompson at 2D+1 (8pts) and will have at least somewhat higher recoil because of that unless the gun in question is very heavy. Exact numbers would again depend on breakpoints.

[BraselC5048]

Well, I certainly didn't realize recoil was based on momentum, not muzzle energy, although I do remember that striking power was based on that (nothing to do with stopping power against people, rather 15" shells against armor plate). Doing the math for a .357 bullet with mass 1 and velocity 10, scaled up to .455 while keeping the same muzzle energy, results in about 37% more momentum, and recoil.

Oh. That means that the .455 revolver should be recoil 4, or too high to be practical. Darn.

Trying to keep the recoil the same, only 1.18x, reducing the bullet from 1.87 times the mass to 1.45 times the mass does the trick while maintaining muzzle energy. Sounds a little weird for something designed to fire APHC ammo, but reducing the density of the non-hard core might well retain armor penetration. Maybe. After all, I'm only trying to keep the pennitration of the smaller caliber, not increase it (increasing the damage type from pi- to plain pi for APHC is a design goal, however). For the SMG, recoil of 2 for similar ammo might make sense.

Of course, most of the time they're actually used in the campaign, they'll be firing "Navy special" HEAT chemical energy rounds, the solution to penetrating body armor with effect in light automatic weapons. Nobody else has it, and the wounds from opponent's PDW's rarely get close to major wound status. (Even 4d pi- doesn't seem scary vs DR 18.) It's a portover from Traveller's Fire, Fusion & Steel, which actually involved calculating the chemical energy (larger caliber being important there).

A while ago, I did a strict kinetic+chemical energy conversion for 7d+1? (2) damage. For consistency's sake, I might change it to 5d+2 (10), keeping the average injury the same as it has been through the campaign, and conveniently avoiding some math in play, since all the body armor likely to be faced is between DR 10 and DR 20, so on a hit, it's just "roll 5d for injury."

[Langy]

Note that weapon weight can significantly reduce recoil. You'd probably want your Recoil number to be proportional to the acceleration provided to the weapon or something similar to that, so it'd be Momentum / Weapon Mass. And bigger caliber weapons do generally weigh more.

[Ulzgoroth]

Originally Posted by Fred Brackin

First thing is that "muzzle energy" is probably not a useful term in dealing with Gurps firearms. It certainly doesn't help me anyway.

Muzzle energy is a big part of determining damage.

[BraselC5048]

Originally Posted by Langy

Note that weapon weight can significantly reduce recoil. You'd probably want your Recoil number to be proportional to the acceleration provided to the weapon or something similar to that, so it'd be Momentum / Weapon Mass. And bigger caliber weapons do generally weigh more.

Good point. The SMG is notable for the age it's made in in that it has a wooden stock and no plastic whatsoever. In addition to looking nice, it also drives up the weight, as does the 71-round drum. A little over 12 pounds with an empty drum, 17 pounds with a full one. Since the nature of it's employment means it will be sitting in a gun locker, even on active duty during wartime, except on short occasions when it's needed, weight isn't a big factor since you don't actually have to carry it around on the march or on patrol. One question - are wood stocks more durable then plastic stocks when it comes to using the weapon as a club?

With the revolver, weights don't seem to vary much. The .357 magnum is already just about the heaviest one out there at 3 lb, and a .44 magnum (much bigger, actually, in terms of sheer bulk, this is clearly a bulk -2 weapon) is only 3.3 lb. Since the .455 Webley actually weighs less then the .357 magnum, it seems that there's not a lot of room for weight growth. It's actually just about the same size as the .455 Webley, yet fires a round as powerful as the smaller caliber .357 magnum. So should it weigh 3.1 lb (looking at the ammo weight, loaded weight would start at 3.1 lb for a weapon that weighs as much as a .357 magnum), or 3.2 lb loaded?
3.2 lb loaded?

[johndallman]

Originally Posted by BraselC5048

Since the .455 Webley actually weighs less then the .357 magnum, it seems that there's not a lot of room for weight growth.

The .455 Webley isn't a high-powered revolver for its calibre at all. As a top-break, it can't be as powerful as a solid-frame gun. British doctrine during its heyday was that you wanted the biggest practical bullet at the lowest practical velocity. This made somewhat more sense before the ban on expanding bullets in warfare, but it persisted after that came in anyway.

So its weight isn't strictly comparable with guns designed to other ideas.

[BraselC5048]

Originally Posted by johndallman

The .455 Webley isn't a high-powered revolver for its calibre at all. As a top-break, it can't be as powerful as a solid-frame gun. British doctrine during its heyday was that you wanted the biggest practical bullet at the lowest practical velocity. This made somewhat more sense before the ban on expanding bullets in warfare, but it persisted after that came in anyway.

So its weight isn't strictly comparable with guns designed to other ideas.

The revolver I'm designing actually is a top-break, with ejector rod, to save reloading time. I figure advances in metallurgy over 70 years make that possible, similar to how they're now making .357 magnums with an 8 round cylinder. It certainly has to weigh less then the .44 magnum, considering that gun is both higher powered and physically larger.

Later models of this gun have a slightly larger, 7 round cylinder in any case. My character, after running out of rounds in his (6 round) revolver with a jammed SMG in one hotly contested action, swears that if he ever gets the money, he'll have one made with a 9 round cylinder (roughly based on the LeMat). Now if the navy bureaucracy can get its act together, the money for destroying all those freighters should fit the bill.