Originally Posted by locutus
That was when the Soviet Union switched from the large, heavy 7.62X39 in the AK-47 to the light, fast 5.45X39 in the AK74. The increase in lethality of the smaller, higher velocity round was very dramatic.
In reality, momentum is a meaningless figure, since it gives equal weighting to mass and velocity, rather than factoring in the square of the velocity. (energy). Even energy has little effect compared to velocity.
It's not that the 5.45x39s were more lethal than the 7.2x39, it's that the foot soldiers can carry far more rounds and the guns can put more bullets in the air. Same logic why the US switched away from the 30-06 in WWII to the .223 -- no one can reasonably argue that a .223 is more lethal than a 30-06, but the combined ammo carrying capacity and rounds-in-the-air seem to be.
As to the physics, momentum is imperfect but it's certainly not meaningless. And "even energy has little effect compared to velocity" ignores the math. Energy is just velocity squared x mass, so for any given bullet energy and mass are interchangeable. As are momentum and velocity. Velocity is no more inherently useful than energy or momentum; all are just mathematical representations of the physics. If a bullet has sufficient velocity for penetration and expansion, then its not at all clear that more velocity is of any benefit. The same cannot be argued bout increasing weight (momentum and energy) and diameter (TKO).
If you take the velocity argument to it's logical extreme, a 1 gr .17 bullet going 10,000 fps would be the ultimate round. Unfortunately that would produce a trivial wound channel, delivering only 0.2 ft-lbs of energy, and would be easily stopped by even light clothing.