Posted by 481: Momentum is conserved even in inelastic collisions.
True, and so I have said. But that does not mean that momentum is the determinant of the distance that an object will travel after being captured in an inelastic collision.
To stop a moving object it is, as you certainly understand, necessary to apply a force to it. And in an inelastic collision, that force involves converting kinetic energy into other forms of energy and/or transferring it to other objects. McPherson said as much.
In stopping a car or truck or a train, friction brakes convert KE to heat, and regenerative brakes convert it to electrical energy.
And guess what:
given a constant braking force, the stopping distance is proportional to the square of the initial velocity--to kinetic energy. When I was young, every college freshman studying engineering performed calculations and experiments showing that.
And penetration is nothing but stopping distance.
Things get a little more complex when it comes to the stopping an airplane on a carrier, where the brakes convert some of the kinetic energy to heat, and some of it is transferred to the arresting equipment; and in addressing the the slowing a vehicle reentering the atmosphere from space, where kinetic anergy is converted into heat, until aerodynamic drag takes over, and perhaps unless a parachute is deployed to shed much of the remaining kinetic energy.
If two billiard balls collide (elastic collision), kinetic energy is conserved as kinetic anergy. But if a car smashes into a deformable building, its kinetic energy is shed in several ways--into heat, into material that is put in motion, and so on. The same thing is true in bullets.
As McPherson said, "kinetic energy is not only not conserved in real collisions, but is transferred into thermal energy in a way that usually cannot be practically modeled." So he decided to come up with correlative models based on momentum. But it would be a mistake to conclude that he ever believed that the stopping distance of a car under a braking force (a far easier thing to model--I've done it) was proportional to momentum. Because it is not, and never has been.
If you haven't read the book, I'd recommend it highly.
I've thought about it. I have long ago accepted his conclusions about what causes wounding and what does not, but I have no interest in reading about his correlative modeling of penetration, however brilliant it may be.
I'm not one of those fires bullets in to gelatin or pays a lot of attention to test results. I've chosen my carry loads, and I do not continue to meditate about how far a single bullet would go into gelatin.
I do stay up with those fine people here who opine about big and slow vs light and fast, but I do not concern myself much about what
a single bullet properly placed would do under ideal circumstances.
So, when it comes to momentum vs energy, I will likely choose low momentum over high, to keep recoil down. And even though a bigger bullet is more effective than a smaller one, there's the issue of round count to keep in mind.
I am not disputing McPherson's assertion that many people have placed too much emphasis on kinetic energy in assessing wounding effectiveness. Full disclosure: until some years ago, I was among them.