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When NATO originally standardized on a caliber, the United States basically forced 7.62x51mm down the throats of the other NATO members, and then just a few years later, the U.S. abandoned that caliber for the 5.56x45 M16 as a general issue infantry rifle.
By the mid-1980s, it had become clear that the U.S. wasn't going back to 7.62x51, so NATO decided to standardize on 5.56mm. One of the issues that 5.56x45 had always had trouble with was being able to penetrate 10ga mild rolled steel (and old M1 steel helmet) at distances over 500m. The NATO requirement called for this ability at 600m. FN managed to come up with a 62gr 5.56mm variant using a 4.7gr steel penetrator that did this reliably they called "SS109." This was ultimately what NATO adopted, though each country has slightly different standards for the so-called standardized ammo. The U.S. version is M855.
With M80 (7.62x51), M193 and M855 ball, the bullet basically traces very, very tiny circles in the air as it flies along its trajectory. Closer to the muzzle the circles start out a bit bigger and then settle down as the bullet gains stability. As 44AMP pointed out, all spitzer shape bullets want to tumble when they hit flesh, it is just a question of when this happens. The momentum of the bullet and the angle of attack of the bullet (where it is at on the tiny circle it is tracing) affect this, but the effect is more dramatic in the smaller 5.56x45 caliber because it has less mass and slows down/yaws faster. As a general rule, M193 or M855 will yaw within about 4-5" of ballistics gel about 70% of the time. 15% of the time, they will yaw earlier and 15% of the time, they won't yaw until after 7" or more of gel.
Being smaller and travelling faster, M193 and M855 both can also be stressed enough to break apart at the cannelure when it yaws (generally above 2700fps, though it can happen at lower speeds). When this happens, the tiny led fragments poke tiny little holes all through the stretched out tissue in the temporary cavity. The combination of stretching and tiny holes causes tissue to detach and creates bigger wound cavities than would be caused just by bullet yaw alone.
However, since where/when the bullet yaws is dependent on the mostly random factor of where the bullet is in its angle of attack (and this randomness gets worse under 50m), this is not considered a desirable trait.
The new M855A1, while it has its own issues, is apparently not dependent on this angle of attack. It yaws and tumbles at a consistent depth every time regardless of its angle of attack. On the other hand, the mostly copper bullet doesn't seem to fragment quite as dramatically as earlier M193 and M855. So while it has better barrier penetration, it tends to create less dramatic wound tracks.*
*I'm not a ballistics expert by any means; but I've been reading this stuff for about 12 years now and have read everything from the original Project SALVO studies up to Fackler, IWBA, etc. This is just a synopsis of what I've read as I remember it. Use at your own risk. Offer not valid in all 50 states.
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