Thread: Replication of ballistic performance @ 400 meters

Originally Posted by veitnamcam

Yep but i don't think it would be a significant change unless the bullet was on the verge of self destruction leaving the barrel.

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I pulled this quote from Bryan Litz off the longrangehunting.com website


I'd like to point out some calculations regarding translational and rotational kinetic energy that are completely open for interpretation.

barrel/bullet:
A typical .30 caliber 155 grain bullet at a muzzle velocity of 3000 fps from a 1:13" twist barrel.

At the muzzle, this bullet will have 3095 ft-lb of translational kinetic energy.
At the muzzle, this bullet will have 7.7 ft-lb of rotational kinetic energy.

When the bullet has slowed to 1/2 it's original muzzle velocity, it will have 1/4 of it's original translational kinetic energy. This will happen somewhere around ~800 yards. At that distance, the rotational speed of the bullet will have decayed as well, but not as much as the forward velocity. The rotational velocity at this point will be about 80% of it's original value. So the numbers after the bullet has slowed to 1500 fps are:

Near 800 yards, the bullet will have 774 ft-lb of translational kinetic energy remaining.
Near 800 yards, the bullet will have 4.9 ft-lb of rotational kinetic energy remaining.

As I said, these numbers are open for interpretation. I can tell you what I think about them, but it's just speculation.

At the muzzle, the rotational KE is only 0.2% of the translational KE. This tell me that practically all of the damage done will be due to the bullet's forward motion, the spin being of little consequence to terminal performance.

At around 800 yards, the rotational KE has grown to 0.6% of the translational KE. This is 3X higher % than at the muzzle, but still an insignificant amount compared to the translational KE.

I am of the opinion that the rotational speed, and rotational KE of a bullet are of very little issue to that bullets ability to do damage on a target. I could be wrong (I've never done the test), but the numbers suggest that there is very little energy in rotation compared to forward motion.

-Bryan
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This is from Kirby Allen (leading american gunsmith making supermagnums)

My only concern with trying to duplicate long range impacts with lower velocity is the fact that RPM levels are dramatically less. Does this make a huge difference, again, with a big game bullet or SMK, more likely then not, not enough to measure in the test.

I do feel that it certainly does effect the stability of the bullet, if not in flight, certainly after penetration. If a bullet, say a 300 gr SMK impacts but begins to tumble or at least yaw and this will not allow the bullet to expand consistantly, at least to my way of thinking compared to a higher RPM impact where the bullet will be far more stable as it impacts and penetrates through the animal.

Again, this is more theory on my part then anything and in most cases its not practical to do actual testing at true long range so this may be the only way to do this other then to get a faster twist barrel to match the RPM levels which again, just is not practical for us to do financially.
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Kirby Allen(50)