Posted by
mavracer:
It [asserting the size of the permanent cavity is directly proportional to momentum, all other things being equal] 's really simple physics, when you define the permanent cavity as what is directly in the path of the bullet you're eliminating all the effects that energy has from the discussion.
I don't think that's a very clear definition of permanent cavity at all, but we do not have to quibble about it. I think we all understand what it is.
But you did not anwser the question. The physics is not as simple as what many people believe it to be. If it were, and if we were talking about crushing and cutting material, we would be speaking of
energy. Energy, or force times distance, is what we measure when we analyze compressing something, cutting something, bending something, crushing something, breaking something, bringing something to a complete stop in many media (e.g., automotive braking, converting kinetic energy to heat (or in some cases these days, to electrical energy). The
change in kinetic energy is defined as
work.
So, in terms of classic physics, one
might tend to conclude that penetration is solely a function of energy rather than momentum, and one could readily demonstrate that penetration that results in a permanent hole in bone or armor or bricks
is in fact a function of
energy.
Quite obviously, it is also energy that causes the bending of parts of the bullet and the increase in diameter.
But when it comes to analyzing penetration in human bodies, it ain't quite that simple. That's because not all of the body is solid. To the extent that some of the body behaves more like an incompressible fluid,
some of the forces on the bullet are in fact a function of
momentum--just as in the case of a canoe paddle in water, or a helicopter rotor blade in air. In those situations, things relate to the changes in the velocity vectors and to the masses of the particles of the fluid.
So, penetration isn't all that simple, and for that reason, permanent capacity is even less so. As Rob Pincus points out, forensic examiners are often unable to determine, though the observation of the permanent would cavity, what it was that caused the wound.
What are the odds you'll have time for additional hits?
Higher if you can shoot more quickly and with control, of course, but if you do not have time,, and if your hits do not happen to damage anything critical, the size of the permanent cavity will not help.
By that logic why stop at 9mm? If the size of the hole doesn't matter 32ACP ball ammo will penatrate 12"+ and is even easier to control.
No one has said that it doesn't matter.