Self-Defense Ammunition: Myths and Magic!

**Scrapper** · Posted On: 5/05/2010 8:47am

Originally Posted by Wing-Kwan-Fu

I can only shout about physics: foot-pounds are units of energy, not force (otherwise what would pounds be?); you can't alter momentum transfer by reducing impulse (the bad guy won't fly back no matter how elastic the collision, unless you fly back the same distance when firing the gun); you consider the speed, not acceleration, of a projectile (or else they would cause undamage, because they only decelerate after leaving the gun); blah blah blah maybe you're a chemical engineer?

Otherwise good article! I'm-a go get a Desert Eagle will 30 rounds of .22

Disconnect between vernacular and strict correctness of nomenclature!

-Foot pounds is how firearms people measure muzzle energy, and therefore force of impact. It ain't right, but it's what happens. The physics part was long and boring enough in this article.

-By definition, Impulse is the change in momentum. If you consider the interaction between the shooter and the discharge of the firearm as a collision, then the difference of mass between the shooter and the projectile ensures that no matter how big the impulse, the smaller mass particle will always be the one to go flying. Not the shooter. The shootee, for the same reasons, will suffer deformation and material failure instead of going flying.

-Saying that HE won't go flying back because YOU won't go flying back is as much an oversimplification as F=MA or referring to ft-lbs as force. Since the shooter can be said to be experiencing an elastic collision and the shootee an inelastic collision.I totally understand what you are saying, though and my dynamic systems and vector calculus is very rusty; but I had to draw a line somewhere with how deep I went into the physics.

-There is no such thing as deceleration. Any delta V is an acceleration by definition. Since vectors have both magnitude and direction, the reduction in speed, and the arcing of the bullet's path are both still considered an acceleration. Always back to vector calculus with you!!!

All good points.

**Robstafarian** · Posted On: 5/05/2010 1:25pm

Scrapper, I think I love you now. All the physics stuff was wonderful (and I honestly wish there was more of it), and the "de Soto" comment gave me the biggest laugh I've had in months.

On the other hand, I hate you for tempting me to buy 500 rounds of .45ACP rather than fix my computer. My lovely 1911 has only had 30 rounds of ball through it (plus one CCI shotshell for a function test), and it hasn't seen the range in almost a year. I knew I'd have this problem if I bought a gun, but I had no idea it would be this bad.

Oh yeah, and you should probably mention that FMJ is sometimes referred to as "ball."

**Aaron_** · Posted On: 5/05/2010 6:49pm

Originally Posted by Scrapper

F=MA is always a slippery slope. It was either write a 10-page paper on Newtonian physics or go with the simple vernacular for effect. ANd you are right...i juxtaposed my elastic and inelastic collision, dammit. Fixing now you bastard.

sorry lol... i read your article about 15 minutes after i left a 3 hour physics lab.

**Wing-Kwan-Fu** · Posted On: 5/05/2010 8:35pm

Originally Posted by Scrapper

Disconnect between vernacular and strict correctness of nomenclature!

-Foot pounds is how firearms people measure muzzle energy, and therefore force of impact. It ain't right, but it's what happens. The physics part was long and boring enough in this article.

-By definition, Impulse is the change in momentum. If you consider the interaction between the shooter and the discharge of the firearm as a collision, then the difference of mass between the shooter and the projectile ensures that no matter how big the impulse, the smaller mass particle will always be the one to go flying. Not the shooter. The shootee, for the same reasons, will suffer deformation and material failure instead of going flying.

-Saying that HE won't go flying back because YOU won't go flying back is as much an oversimplification as F=MA or referring to ft-lbs as force. Since the shooter can be said to be experiencing an elastic collision and the shootee an inelastic collision.I totally understand what you are saying, though and my dynamic systems and vector calculus is very rusty; but I had to draw a line somewhere with how deep I went into the physics.

-There is no such thing as deceleration. Any delta V is an acceleration by definition. Since vectors have both magnitude and direction, the reduction in speed, and the arcing of the bullet's path are both still considered an acceleration. Always back to vector calculus with you!!!

All good points.

-I just didn't like seeing "ft-lbs of force" because muzzle energy only tells you the force of impact if you know exactly how quickly the bullet decelerates on impact, which you mentioned varies widely. And even then it's several steps of math away...

-You're right about the impulse-momentum relation (my bad). Except for "the longer the force is applied, the smaller the impulse". If the bullet comes to rest, the impulse imparted is the same regardless of how quickly it decelerates (I = the entire momentum of the travelling bullet). What you meant to get at was that the peak force is smaller if the impulse is more spread out over time.

-So, you're right that, depending on the collision, the shootee might receive less impulse than the shooter (if the bullet over-penetrates), or as much as double (if he's Superman, and the bullet bounces backwards at full speed). But if the bullet comes to rest in the shootee, then the momentum change is identical for both parties. I just meant that he stays put because of his greater mass, as you mentioned above, not because of the inelasticity, which you implied in the article.

-About acceleration, I just got muddled in your mixing of talk about velocity and acceleration. I thought you were talking about the acceleration of the bullet in the barrel and air mattering, instead of just the acceleration caused by the collision.

-Let's discuss the quantum-tunneling effects of teflon-coated tachyon bullets!

**Aaron_** · Posted On: 5/05/2010 10:00pm

Originally Posted by Wing-Kwan-Fu

-I just didn't like seeing "ft-lbs of force" because muzzle energy only tells you the force of impact if you know exactly how quickly the bullet decelerates on impact, which you mentioned varies widely. And even then it's several steps of math away...

-You're right about the impulse-momentum relation (my bad). Except for "the longer the force is applied, the smaller the impulse". If the bullet comes to rest, the impulse imparted is the same regardless of how quickly it decelerates (I = the entire momentum of the travelling bullet). What you meant to get at was that the peak force is smaller if the impulse is more spread out over time.

-So, you're right that, depending on the collision, the shootee might receive less impulse than the shooter (if the bullet over-penetrates), or as much as double (if he's Superman, and the bullet bounces backwards at full speed). But if the bullet comes to rest in the shootee, then the momentum change is identical for both parties. I just meant that he stays put because of his greater mass, as you mentioned above, not because of the inelasticity, which you implied in the article.

-About acceleration, I just got muddled in your mixing of talk about velocity and acceleration. I thought you were talking about the acceleration of the bullet in the barrel and air mattering, instead of just the acceleration caused by the collision.

-Let's discuss the quantum-tunneling effects of teflon-coated tachyon bullets!

Scrapper was correct in his statement about ft.-lbs being the measurement of force commonly used to describe bullets. Sort of how you could measure forces split into 3space vectors in Newtons. it sounds like what you were talking about was pressure, or force divided by an area, resulting in a Psi measurement.

Also, based on the shape of the bullet, the air temperature, the average density of the target, and in long distance cases, the rotation of the earth, it is entirely possible to calculate how much force would be exerted on the target. however, this would be very complicated to the point where it's not even worth calculating.

Scrapper was also correct in saying that there is no such thing as deceleration. acceleration is a vector quantity, having both magnitude and direction. what you are talking about is acceleration of a negative value based on the idea that in your shooting coordinate system, the bullets flight path would be in the positive i direction.

And acceleration is the derivative of velocity with respect to time, making Scrapper's statement about acceleration being a change in velocity over time correct.

Don't even get started on quantum mechanics...

**Wing-Kwan-Fu** · Posted On: 5/06/2010 12:05am