shiv wrote:Are you saying stopping power (momentum)will be the same at 2x the range for a bullet twice as heavy, but the KE would be the same for 5.56 and 7.62 at 1x and 1.4x range respectively?

Why do I feel that KE is more related to penetration than mere momentum? Like I said I am missing something.

Yes thats what I am saying. lets say the two bullets start with the same velocity, v and m & 2m are the weight of the small & bigger bullet. If the speed reduces linearly with distance and x is the unit where speed reduces by half, ie for every x distance travelled speed becomes v/2:

Range ----- Big bullet Momentum ----Small bullet momentum ----- Big bullet Energy ----- small bullet energy

0 ---------- 2mv ------------------- mv ------------------------ mvv ----------------- mvv/2

1x --------- mv ------------------- mv/2 ---------------------- mvv/4 --------------- mvv/8

1.4x ------- mv/1.4 --------------- mv/2.8 -------------------- mvv/8 --------------- mvv/16

2x ------- mv/2 ----------------- mv/4 ---------------------- mv/16 --------------- mv/32

You see, if velocity falls linearly, momentum would fall linearly too, but KE would fall squarely. But since the big bullet has twice the weight compared to the small one, it would have two times momentum at same range, hence same momentum at twice the range. However, in case of KE, the big bullet starts with 2 times the energy of the small one, but energy falls squarely to the velocity, hence if the velocity falls by half, the energy falls by 4 times. Hence energy drops faster than momentum. Hence the same energy at 1.4x distance.

With regards to penetration:

Generalized Newtons second law,

F = m * a ( m = mass & a = acceleration )

=> F = m * dv/dt ( where v = velocity , acceleration = dv/dt, ie rate of change of velocity)

=> F = d(mv)/dt ( where mass remains constant)

=> F = dP/dt ( since momentum P = m*v)

So as you see force is nothing by rate of change of momentum, which is actually what Newtons second law states. Its this force which would determine the impact power of the bullet. You are right that penetration & tissue damage would be based on the actual energy of the bullet. The tissue damage is what would kill you, but the initial force is what would make you loose your balance & fall to the ground or stop you on your path if you are running towards me. These are two different things. I can slowly stab you with a knife and do tissue damage, that's not going to knock you to ground but can sure can kill. Also I can punch you hard, with out causing tissue damage, but sure can knock you down, with bruises as bonus. You get it, one knocks the person down to the ground & the other kills him.