Momentum is a pretty good indicator of how far a bullet will travel inside a target,
Actually, I believe that with a little study, you will come to the conclusion that, in an ideal fluid (probably including gelatin), energy is a far better indicator, given the same bullet construction, design, and sectional density. Simple physics. Essentially the same as drag and dynamic braking.
...and what sorts of obstacles it will be able to break through before it stops.
At that point we depart from simple physics. It may well be (and it stands to reason to me, but you have the experience to judge) that a heavier bullet moving more slowly, with the same energy but with higher momentum, will be able to go through bones more effectively. I don't know, but the guys with the big guns in Africa might.
When organs explode (or game explodes), that takes a good deal of energy.
Good point.
Below a particular threshold of bullet velocity (and energy), energy means very little. Bullet momentum means more, because it is a better indicator of the bullet's behavior on target.
Second point first: One more time, unless the issue is penetrating bone, the
only way momentum enters into the picture (other than in terms of recoil) is in
how much velocity is imparted to the target--how much the target is "nudged," in your words--and I believe that is meaningless, and you've said so too.
Regarding energy: Assuming the same sectional density and bullet construction, energy will drive penetration, except possibly in the case of bone. As I understand things, penetration, and wound channel diameter will be critical.
...a mammal like a man or a deer can withstand a shockwave up to a certain threshold which is far above the energy of the hottest 9mm available.
Could be, and it makes sense. Is there empirical data to prove that the shock is not debilitating?
You sure don't have to explode any organs to do damage.
You can knock a man out with a blow to the ribcage without breaking anything. How much energy is involved? Might the effect of an expanding bullet hitting near an organ or nerve be temporarily damaging?
I won't argue the point when it comes to a 9MM, but I really don't know, and I do think that a hot .357 from a long barrel is probably a different story.
By the way, I retired my 9MM some time ago, because it will only function reliably with ball ammunition. Neither the energy nor momentum in a bullet that has passed through the target is of any use to me.