Momentum is not a driving force -- energy is. Momentum is only a measure of motion's relative persistence.
I was using a colloquial phrase, not a direct description of the physics.
Any object in motion owes its momentum to the energy that has put it into motion.
Momentum is a product of mass and velocity, not its energy.
Look, whether we attribute it to energy, momentum, or circumvadial fertropian mertropathy of the third order, this is basic: a heavier, faster moving object will impact with greater force than a lighter, slower moving object. This applies to cars, trains, and, yes, bullets.
Yes more mass and more velocity means more energy too... But with bullets, heavier means slower... (when talking about the same caliber/loading) So you don't get heavy and fast... you can have heavy and slow or light and fast. (well you can do light and slow too if you wanted)
I disagree. Momentum tracks most closely with penetration
This...
During one of the many debates on bullet energy and penetration... I went over the physics of bullet penetration... Many people (most actually) disagreed with me.. And while I didn't go so far as to do calculations, and I didn't explain things as clearly as I could have... I knew I was on the right track with it. While I enjoy science and scientific inquiry, and am good at math (yet hate doing it) I am not an expert in the field of terminal effects.
So, I did some digging and research and found that all the experts that tested terminal performance professionally agreed with me... they also did a much better job at explaining the effects and why.
People claim the higher energy numbers of the 40 make it better, when energy is not that important as I explained above.
Even in rifle calibers, total energy is not the end all be all. In a rifle round, temporary cavity/cavitation is a big component of the bullet's terminal effects. They do not rely on penetration and direct tissue destruction alone, like handgun calibers.
Cavitation is a product of velocity, not energy. Two objects of the same size, shape, and velocity, but of differing mass, will produce the same cavitation in a fluid. (The added momentum of the heavier object will allow the object to travel farther through that fluid though, so it will create cavitation over a longer distance)
Lightweight and fast projectiles can do a whole lot of damage... just in a shallow area. They tend to lose out to heavier stuff on larger game due to that lack of penetration and deep damage. Heavier bullets hold together longer, and shed velocity less quickly, allowing the cavitation damage to reach deeper. (a product of momentum) Not to mention the benefits simple deep penetration gives as well.
Energy is really a byproduct of other factors that have/give the desired effects. Its not a useless figure, but it is not the deciding factor for performance. Its just an easy to understand figure for the layman, and provides an easier way to compare different loads.
40 is a good round, and it has some benefits in some areas over the 9mm... I believe barrier performance is better if I am remembering correctly, and it does have a little larger expanded diameter. It may also have some other fringe advantages... But those advantages are not very large, and have little practical effect on performance when the round hits a target.
(Really the argument that a millimeter or two added width to expansion makes a difference is silly... "That 9mm barely missed the heart, should have used a 40, it would have expanded bigger"... And what? Missed the heart a little less? Nick the heart a little, add a little extra bleeding?... Its not like it would have expanded enough to do real damage to the heart... the bullet isn't going to jump an inch to the left or anything.)
For the most part and in most situations, the 9mm is just as good. It also has advantages like cost, more capacity, and lower recoil. (I personally can tell the difference between them when fired from the same platform, and my fiance definitely can)
It is those advantages, which I think are more practical concerns, which put the 9mm in the position of being a better choice overall.
I will not begrudge anyone for choosing 40.. but I don't feel it does anyone a service, especially new shooters, to make claims about the 40 which are not true, relevant, or have any practical effect on terminal effectiveness.
