A physics question

[kallend 1,623]

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From the general information on small arms ballastics I have read in relation to target shooting and hunting. Momentum seems to be the more accepted indicater of knock down power.

A high pressure 9mm round will have more kinetic energy than a regular 45 ACP, although the 45 will most likely knock the person over who get tagged by one, knocking someone over with a 9mm is less likely, according to the old guys at the range I go to.

The weight of a 45 round is around twice that of the 9mm. This is an area where practical results differ from what science would expect. The study of small arms reveals a number of these unexpected peculararities. Hunters, LEOs, and the military have noticed this is the case through actual experience, rather than through scientific prediction.

When a gun is fired, conservation of momentum requires that the bullet's momentum is equal and opposite to the gun's momentum. The bullet then delivers this to the target. So the target and the shooter each receive the same dose of momentum.

The same is not true of kinetic energy. The bullet carries away the bulk of the KE.

If you can show me a documented situation in which the law of conservation of momentum is violated, I will concede your point.

...

The only sure way to survive a canopy collision is not to have one.

[kelpdiver 2]

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The weight of a 45 round is around twice that of the 9mm. This is an area where practical results differ from what science would expect. The study of small arms reveals a number of these unexpected peculararities. Hunters, LEOs, and the military have noticed this is the case through actual experience, rather than through scientific prediction.

This may be more about the higher velocity, narrower profile 9mm slug tending to go through, so not all of the energy or momentum is imparted to the target. BTW, I see them having about the same energy - 125grain @ 1150fps versus 230grain @ 850fps. Momentum is clearly higher for the .45.

Neither bullet is going to knock over the guy.

[Avion 0]

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Momentum is clearly higher for the .45.

That's mainly my point. One of the other interesting particularities of small arms ballastics is that on different days where all measurable parameters, temprature, pressure, wind etc, are exactly the same, rounds from the same lot of manufacture fired through the same gun will display different degrees of accruarcy. That is to say one some days the same lot of ammo will produce tighter or loser groups than on other days. This remains to be explained.

As for the 'knock down' power, I am motivated to do some of my own experiments involving weights traveling at specific velocities impacting a board or some type of protection with a person on the other side, and gather subjective impressions of what double the impact or knock down power is compared to actual mass and velocity. Should be easy enough to do. Might take some time to get around doing though. My expectation is that double the knock down power as reported by the reciepent will equal double the momentum, and the double the kinetic energy will be less likely if at all to be reported as double the impact or knock down power.

Maybe I can put some kind of gauge or get somebody to stand behind a piece of 1/4 steel plate and fire a 9 and 45 at 'em, and then see what they say about comparative impact

I do know that 45s are prefered over 9s on pin nights, where the goal is to knock bowling bowl pins off a table. 9s are less reliable in getting the pins off the table.

[Griff69 0]

Wow, a topic here on which I'm not completely ignorant!

The point most often missed in this debate is that "knock down power" or "stopping power" is a fallacy. You can measure the KE or momentum of a projectile, but as was already pointed out, this measure alone means nothing. Any handgun that can, by virtue of momentum alone, knock someone over will also knock over the shooter. Whether the target goes down is impacted more by the shape and composition of the bullet, whether it deforms, how much damage it causes, and most importantly, psychological conditioning. Most people fall down when shot because they think that's what is supposed to happen.

And personally, I prefer .40 for pins...

Chris

"Democracy is two wolves and a sheep voting on what to have for dinner. Liberty is two wolves attempting to have a sheep for dinner and finding a well-informed, well-armed sheep."

[Avion 0]

As far as general accuracy, I've noticed I do better with 9mm and 45s than 40s and 10mm. 40s are a little sharp on the recoil for me, and 10s are just too much. Bullet construction does matter, full metal jackets may go right through the target thus imparting less than 100% of its momentum to it, whereas a hollow point will often transfer 100% or close to it.

My point also include the fact that in order to double the momentum of an object, it's speed must be increased by a factor of 2 that is twice the original speed. In order to double the kinetic energy of an object is speed must only be increased by a factor of 1.4 that's less than one and a half times the orignal speed.

I'm rather interested in the results of my little experiment, if can ever get around to doing it.

Cheers

[billvon 2,400]

>Maybe I can put some kind of gauge or get somebody to stand
> behind a piece of 1/4 steel plate and fire a 9 and 45 at 'em, and
> then see what they say about comparative impact.

There's an easier way to tell. Measure the recoil. The amount of momentum transferred in the recoil will be exactly the same as the amount of momentum transferred to the target, providing the slug doesn't 'bounce' away or anything.