Wednesday, April 2, 2014

Momentum Has Nothing to Do With Stopping Power

There is a major misconception that pops up often in discussions of small arms: That momentum reflects in some way the terminal effect that a projectile has on a human target. It seems to be standard in the gun journalism industry when evaluating new calibers for game or war to test them against steel poppers, implying or even outright stating that this informs the terminal effect of the round. Even Larry Vickers, to whom I am not even close in terms of experience, says in this video about PDWs that the low momentum produced by the 4.6x30 round - making it unable to knock down the steel target - is a "clue" to low terminal performance.

Now, I have little expectation that a 4.6x30 round, which produces about 540J from the MP7, will perform much better against a human target or gel block than a 9mm JHP or even FMJ. It may more consistently perform after penetrating ribs, but in general, the round is limited in its effectiveness by its low muzzle energy, and its ability to deposit that energy in the target (link starts a download). However, is what Larry says true? Is low momentum a "clue" that a round might not have very good terminal effectiveness? Well, I don't really think so. Sure, a cartridge with marginal terminal effectiveness, like the 4.6x30, might have low linear momentum. However, a cartridge like .45 ACP, which in hardball form produces no greater energy than the 4.6x30, produces more than two and a half times the linear momentum; comparable to the much, much more effective 5.56mm round, in fact.

Because of all the variables involved in the problem of terminal effectiveness against human targets - including the target's mental state, the perceptions of the shooter, and most important, the location of the hit - it can be difficult to say what is and is not relevant to the total sum of terminal effect. However, momentum is one metric that can be discounted entirely. Consider that when a gun fires, it creates a force going in two directions, the bullet and gas going forward, and the firearm itself going backward. This force acts on both bodies over the same length of time - that is, however long (and a little after, due to muzzle thrust) the bullet is in the barrel. Because the forces pushing the bullet and gas out the barrel, and pushing the gun backwards against the shooter's shoulder are equal and act over the same length of time, the momentum of the sum of the bullet and the gas propelling it, and the rifle recoiling, is the same. This means that the momentum of the rifle recoiling into your shoulder as you fire will always be greater than the momentum of the bullet as it hits the target, for two reasons. First, because the gases escaping from the muzzle account for momentum lost, and because the bullet loses velocity - and thus momentum - as it flies downrange, whereas the rifle doesn't have to travel to recoil into your shoulder.

However, we observe as the unspoken first law of shooting that guns have a deadly end, and a non-deadly end. If momentum informed the terminal effect of a weapon against living targets, we'd all be dead fools.

So remember, the next time you're shooting silhouette targets with your .45 ACP 1911 and they fall with a satisfying "clunk" to the ground, the only game the momentum of that 230gr hardball ever felled were made of AR500 steel.

8 comments:

  1. "the only game the momentum of that 230gr hardball ever felled were made of AR500 steel"

    I think I'm misunderstanding this sentence, are you saying that .45 acp hardball hasn't ever been used to take game animals?

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    1. No. I am saying that momentum is a negligible factor in its terminal performance against living targets.

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    2. I thought I was goofing it up. Now that I reread it again it is clear.

      As to the momentum issue, I am fully in agreement. Thank you for the great article.

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  2. sorry but i disagree . throw a tennis ball at me and it will hurt . fire a tennis ball at me from a cannon and it will probably kill . how is momentum a negligible factor again?

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    1. The tennis ball cannon in your example produces higher energy, as well as higher momentum.

      A 9mm projectile fired from a handgun produces about as much momentum as someone stifly bumping in to you. It produces about as much energy as one of Bruce Lee's punches. Given that this is true , you can conclude that momentum has little to no bearing on lethality.

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  3. oh man you have to approve my comments before they appear? i just found this blog and it looked cool before i read that. thanks but im out of here.

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    1. I am sorry you do not appeciate my comment approval policy.

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