Why Load Lighter Bullets in 10mm?

[10mm-Admirer]

I watched some penetration tests on YouTube last night and think I get it now. It is amazing how much of a difference 10-25 more grains of bullet weight can affect penetration depth! It doesn't seem like it would make that much of a difference but it does.

[my_old_glock]

Here is what is happening from a basic physics view. There is actually a little more happening, and I would have a hard time explaining it because I didn't finish engineering school.

Momentum is what keeps an object going in the same direction. The more momentum an object has, the harder it is to stop, or the more external force is needed to alter it's course (slow down). Momentum is equal to mass times velocity (p=m*v). Power factor in shooting competition is a basically momentum.

Energy is required to do work. Work is done opening a hollow-point bullet. Energy is equal to one half mass times velocity squared (K=(1/2)m*V^2). You will notice that energy (K) has mass (m) and velocity (V) as the only two variables - just like momentum. As work is done opening up the hollow-point bullet, energy is used up, so either mass (m) or velocity (V) has to change. Since mass doesn't change, velocity has to change.

The medium that the bullet will do its work in (body tissue, water, etc) is also a factor, and must be considered. Take two identical hollow-point bullets and two identical buckets of water, and do the following:

1) Hold one bullet just over the surface of the water, with the hollow-point facing the water, and let it go. You will notice that it drops to the bottom, and the bullets shape doesn't change: It looks new.

2) Load the other bullet in a cartridge and fire it into the other bucket. You will notice that the bullet deforms and opens up.

The difference with bullet and bucket #2 is that the medium did not have enough time to move out of the way. The medium did work on the bullet and opened it up. I do not know enough about fluid dynamics to explain it better.

If a hollow-point is not traveling fast enough, the medium has time to move out of the way, and it doesn't' open up. If it doesn't open up, there is less resistance (external force) slowing the bullet down. If the medium moves out of the way there is also less resistance (external force) to slow down the bullet. With less resistance (external force) the momentum is not reduced, and the bullet travels farther.

For every action there is an equal and opposite reaction. The faster a bullet travels the more resistance it sees in the medium. The more resistance it sees in the medium the faster it opens up and the faster it slows down. The shorter the distance it takes to stop the greater the impulse on the bullet. The greater the impulse on the bullet the greater the impulse on the medium (that's what makes water jugs explode). There is a possibility that the impulse can create a shock to internal body parts (there are no tests to confirm this, and a lot of people don't believe it happens. It is sometimes called "energy dump"). The impulse energy is probably what causes the tissue surrounding the wound track to tear.
If you were to wear one of those ceramic bullet proof chest plates and someone shot you in the chest near your heart with a 50 caliber machine gun, your heart would probably rupture even though the bullet didn't penetrate the plate.

If your hollow-point bullet is not going deep enough into the medium, try slowing down the velocity. If the the hollow-point bullet is going too deep in the medium, try increasing the velocity. I know that sounds counter-intuitive.


I hope that makes some sense.

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[The_Shadow]

My_Old_Glock, Nice write up!  Yes, it does make sense to me.  The physics, the dynamics and all the other factors, can and will affect the bullets of choice as they encounter the intended target or barriers.

One thing is for certain, shot placement is of the utmost importance, where the work of the bullet can unleash its potential.