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Old 02-13-2010, 01:32   #379
glock20c10mm
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Quote:
Originally Posted by uz2bUSMC View Post
I have to say (in reguard to N/Apower ) that I do not believe that you possess the knowledge of retarding forces that you believe you do. If you did, you would EASILY understand how a fmj would not impart it's energy towards the retarding forces the way an expanding projectile would, period. It's not even a discussion point but yet, here we are.
You're right, he doesn't get it. Maybe now he will. My fingers are crossed. Now I see he's posted back as I've quoted you.



N/Apower,

Here is some info that should help you understand and answer to your curiousness -

The equation for JHP handgun bullets with 100% mass retention is -
p = (5*E)/(pi*d)

p is the peak pressure wave magnatude on the surphase of a 1" diameter cylinder centered on the wound channel (in psi). E is the impact energy (in ft-lbs) and d is the penetration depth (in feet).

If a JHP bullet fragments then generally whatever % the bullet fragments is the same % you need to add to the PBPW originally figured for nonfragmentation.

For FMJ handgun bullets the equation changes to a reasonable approximation of -
p = (3*E)/(pi*d)

For FMJ rifle bullets there is much more variation because some tumble deep and some tumble at shallow depths and some fragment. The retarding force profile (the more retarding force the greater the PBPW) is dominated by the depth at which a FMJ rifle bullet tumbles.

An FMJ rifle bullet which does not fragment and tumbles late in the penetration (10" or more) will have a peak pressure wave comparable to the formula for FMJ pistol bullets.

An FMJ rifle bullet which does not fragment and tumbles early (first 4") will have a peak pressure wave comparable to the formula for JHP handgun bullets.

You might wonder why PBPW goes up with bullet fragmentation. This involves a bunch more math which I can post if you like, but I don't see that it's necessary. What I do understand is the basic principal which I believe will be simple for you also once you simply basically understand the basic equations above for equating PBPW.

If it is necessary for you, maybe this will help, and it's about as far into as I'ld prefer to get. If kinetic energy and penetration depth are equal, bullets that fragment create a larger pressure wave than bullets that retain 100% of their mass because the average penetration depth is shorter than the maximum penetration depth. Less penetration depth with equal kinetic energy = higher PBPW.


Also, not to rush you, but I did ask you specific questions you haven't answered in your last post. I'm hoping to hear from you on them unless you're simply acknowledging you were wrong by ignoring them.


Good Shooting,
Craig
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