What Is This Bullet For?

[warellis]

Regarding body armor, wasn't the Belgian SS109 5.56mm NATO round adopted in place of the M193 5.56mm round due to having better armor-piercing characteristics, at some expense of the impressive fragmentation of the original M193 bullet?

And yasn't it been found that 5.56mm NATO ball is actually better tyan 7.62mm NATO ball at penetrating like level 3 trauma plates due to higher velocity?

Anyways, how does GURPS treat the newer M855A1 5.56mm NATO round and M80A1 7.62mm NATO rounds the US is currently equipping its troops with?

[Varyon]

Originally Posted by warellis

Regarding body armor, wasn't the Belgian SS109 5.56mm NATO round adopted in place of the M193 5.56mm round due to having better armor-piercing characteristics, at some expense of the impressive fragmentation of the original M193 bullet?

Going off of Wikipedia, the SS109 was designed with armor penetration at range in mind. It was probably ultimately chosen because of this. Its armor penetration and reduced fragmentation are probably both below GURPS resolution.

Originally Posted by warellis

Anyways, how does GURPS treat the newer M855A1 5.56mm NATO round and M80A1 7.62mm NATO rounds the US is currently equipping its troops with?

The first sounds like it has comparable wounding characteristics to the original round, and comparable armor penetration to the SS109. Still below resolution. The M80A1 apparently uses a similar design, so probably again below GURPS resolution.

[fredtheobviouspseudonym]

All quite variable.

Certain of the following information is from "The American Rifle," written by a journalist rather than ballistician, but which also has a decent set of citations.

And from dim memories of Ezell's "The Black Rifle: M16 Rifle Retrospective".

The original AR-15/M-16 round had a 55 grain bullet (c. 3.5 gms) and was fired from a barrel with a very low rate of twist, c. 1 turn in 18 inches IIRC. While this minimized accuracy at long range and reduced armor (and other material) penetration) it really increased tissue damage as the bullet would generally tumble following a very brief penetration of flesh. This caused the stories about the M-16's alleged incredible lethality in the early days of Vietnam.

If a bullet is rotating relatively slowly on impact with something hard (read armor) it tends to tumble instead of maintaining its orientation and attitude with regard to the surface it strikes. If it tumbles it dissipates energy in the tumbling (instead of using that energy to punch through that surface). In addition it contacts more of the material's surface (hitting somewhat sideways instead of straight on) again dissipating its energy over a larger surface area. This again lessens the chance of penetration.

So in the later 1970s the US Army developed a longer bullet and used barrels on the M-16 with a tighter rate of twist -- c. 1 in 8 or 9 inches. So the lethality dropped but the chance of the bullet penetrating helmets, body armor, or cover (shoot through the tree) went up.

[Anthony]

Originally Posted by fredtheobviouspseudonym

If a bullet is rotating relatively slowly on impact with something hard (read armor) it tends to tumble instead of maintaining its orientation and attitude with regard to the surface it strikes.

There's no practical rate of rotation that will prevent bullets from tumbling on impact with any significant mass. The issue for wounding is not whether the bullet tumbles, it's whether the bullet fragments. The 62 grain bullets are both more durable and slower than the 55 grain bullets, and thus they fragment less.

[warellis]

http://www.thefirearmblog.com/blog/2...-epr-glorious/

M80A1 uses a lighter 130gr projectile than its nought mark 147gr predecessor, M80. The construction of the new projectile is the same as the 5.56mm M855A1, with a copper “slug”, hardened steel penetrator, and both wrapped in a gilding metal jacket.

During filming of this video, William was just working up loads to figure out which would best replicate the Army’s M80A1, which he believed to produce 2,950 ft/s muzzle velocity. In fact, according to PEO Soldier’s 2013 Ammunition Portfolio, M80A1 produces 3,050 ft/s at 78 ft from a 24.8″ M240 barrel, which would mean roughly 3,050-3,060 ft/s at the muzzle from a 22″ barreled weapon, such as an M14. William told TFB that high speed footage of M80A1 in partnership with Aimed Research is in the works, so stay tuned!

http://www.thefirearmblog.com/blog/2...new-round-gel/

As for tumbling, hasn't that been disproved and in reality it has been found that bullet fragmentation is what kills?

[Varyon]

Assuming the fragmentation and penetration we see in those videos is typical of performance in flesh, the actual performance of these bullets may be something rather complicated in GURPS terms. Basically, you're looking at an armor penetrator strapped to a frangible round. I'd eyeball it as follows: the bullet simultaneously has the performance of an AP (not APHC) bullet and a frangible bullet, inflicting the greater injury of these options. So, with a 5d pi weapon, you'd go ahead and roll 5d. Multiply apply an armor divisor of (0.5) and record the injury as though the weapon were pi+. Multiply damage by 0.7, then apply an armor divisor of (2) and record the injury as though the weapon were pi-. Use the greater result. So, for 5d pi against DR 12 armor, let's say you roll 26 damage (getting very lucky). The fragments would do (26-24=2; x1.5=) 3 injury, while the penetrator would do (26*0.7=18; -6=12; x0.5=) 6 injury - use the 6 injury.

To simplify things, you'll typically just treat it as fragmentation ammo against lightly armored foes, while against those with DR of (dicex2) or greater you treat it as AP ammo. Foes with DR between (dice) and (dicex2) are more finicky and might need you to calculate both.

[Anthony]

I wouldn't do anything exotic about the M855A1 EPR; GURPS in general doesn't worry about the details of bullets. Assuming its nominal penetration of 3/8" is correct, it's got an armor divisor of 1.5 and probably does normal pi damage.

[Fred Brackin]

Originally Posted by warellis

As for tumbling, hasn't that been disproved and in reality it has been found that bullet fragmentation is what kills?

No, tumbling is real and all boat-tailed spritzers tumble. Normally only once. With supersonic impact velocities and light jackets most rifle bullets will fragment at the canellure because they have tumbled i.e. they break in the middle while they are trying to go sideways.

Slower and thicker jacketed bullets may not fragment at all and only the extremely high velocities and ultra-light jackets of real "varmint" rounds (and the 5.56 doesn't really qualify) will fragment before tumbling.

Some missed Armory Skill rolls while developing loads can cause such bullets to fragment while still in the barrel. You really only need to have the bullet survive intact to that point.

[DouglasCole]

Originally Posted by Fred Brackin

No, tumbling is real and all boat-tailed spritzers tumble. Normally only once. With supersonic impact velocities and light jackets most rifle bullets will fragment at the canellure because they have tumbled i.e. they break in the middle while they are trying to go sideways.

Slower and thicker jacketed bullets may not fragment at all and only the extremely high velocities and ultra-light jackets of real "varmint" rounds (and the 5.56 doesn't really qualify) will fragment before tumbling.

Some missed Armory Skill rolls while developing loads can cause such bullets to fragment while still in the barrel. You really only need to have the bullet survive intact to that point.

The tumbling thing came from a real observation from early weapons where the twist rate in the barrel was not high enough to stabilize the bullet, leading some to tumble in flight. This was a fatal error and quickly fixed. The other source of this is some early illustrations - grossly exaggerated for illustrative purposes - of in-flight yaw and precession.

[Fred Brackin]

Originally Posted by DouglasCole

The tumbling thing came from a real observation from early weapons where the twist rate in the barrel was not high enough to stabilize the bullet, leading some to tumble in flight. This was a fatal error and quickly fixed. The other source of this is some early illustrations - grossly exaggerated for illustrative purposes - of in-flight yaw and precession.

I meant tumbling in terminal ballistics i.e. in flesh. That seemed to be warellis' context as well. Of course you don't want and with odd exceptions don't have bullets that tumble in mid-flight.