True, but one also needs to consider how much energy is expended by going through various vehicle panels/upholstery/components/etc. Does the round still have enough energy by the time it strikes the target? A piece of cardboard will show a hole even with a weak hit.
So true. A bullet has so much kinetic energy, and it loses it over time after it exits the barrel. It loses more if it hits something along the way. A bullet with more kinetic energy AND greater cross sectional density in a somewhat weighty bullet will penetrate more than one without these attributes. It is all in the numbers. Using Newtonitan physics, it isn't that hard to show the difference between the 9mm and .45, for instance.
In easy lay terms, say a 4" .357 mag is fired at a car (door, window, take your pick). What comes out the other side is, for all practical purposes, a .38+P or 9mm. You lost that KE when it hit the intervening material.
No wonder that a large, slow bullet with poor cross sectional density is a poor performer when called upon to penetrate intermediate media like sheet metal, walls, and glass. I've heard of .44mag not penetrating car doors at close range. They guy was firing very heavy slow loads (240+? 300?). It would go in one side, but he was shocked (I was too) that it didn't penetrate. He shot it with a 9mm, and it went in one side and out the other.
Another issue is bullet construction. At very high KE, like with an M4, the bullet can and will disitigrate in a spectacular fashion when it hits hard materials. Believe it or not, it is a crapshoot as to whether the M855 ball will penetrate a cinderblock! It will go in one side, but not the other. It takes multiple rounds to break one down.
Lots of variables in a gunshot. VERY dynamical system, which makes it so hard to say for sure what works well and what doesn't.
Personally, I think the 10mm is the ideal weapon, followed by .357 magnum. You can load heavy and slow, or light and fast, all without losing the superior cross sectional density to other comparable loads.
