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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet RPM and performance
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<blockquote data-quote="RockyMtnMT" data-source="post: 2525588" data-attributes="member: 7999"><p>Terminal performance is more dependent on rpm than that. The same bullet fired at the same velocity from a faster twist will out penetrate the slower twist. There is terminal stability that is different than ballistic stability. As a bullet penetrates the rpm's degrade rapidly. At some point during penetration the retained shank will run out of rpm. When that happens it will get to a point of instability and then tumble. Same bullet fired with higher stability will straight line penetrate farther before it tumbles. If a bullet is fired with just enough stability to fly ballistically but not enough for proper terminal performance you will see a bullet failure. If the bullet has a bit of yaw or wobble on impact and the nose is not point on at impact then it will not get fluid into the hollow point and expand properly. This is when you see bullets recovered that look like a banana. This can result in pretty destructive results if the bullet tumbles through on the line of travel. It can also result in a bullet that turns drastically and never gets to the vitals. It can also result in a bullet that does nothing and pencils. We learned this when we first started impact testing bullets in the beginning of marketing Hammers. We were low velocity testing to confirm proper performance at 1800 fps. Launched a bullet at 1800 fps and recovered it looking like it could be loaded again. The only distortion was engraving from the rifling. Very disappointing as this is our advertised min. We decided to increase the vel to find out the min for this particular bullet, but could not get any more powder in the case. We had another rifle of the same caliber with a larger case so we grabbed it. First launch was right at 1800 fps. I remember thinking " That was a wasted shot". Went and dug the bullet out of the media to find a perfectly deformed shank. Hmmm... The only diff was barrel twist. Lots of lightbulbs went on at that moment. Ran the stability numbers and the first built was about 1.2 sg and the second bullet was over 1.5 sg. </p><p></p><p>Just because you can get a bullet to an animal accurately does not mean the bullet will perform properly. Marginally stable for ballistics can shoot extremely accurately but significantly increase the odds of failure to expand.</p></blockquote><p></p>
[QUOTE="RockyMtnMT, post: 2525588, member: 7999"] Terminal performance is more dependent on rpm than that. The same bullet fired at the same velocity from a faster twist will out penetrate the slower twist. There is terminal stability that is different than ballistic stability. As a bullet penetrates the rpm's degrade rapidly. At some point during penetration the retained shank will run out of rpm. When that happens it will get to a point of instability and then tumble. Same bullet fired with higher stability will straight line penetrate farther before it tumbles. If a bullet is fired with just enough stability to fly ballistically but not enough for proper terminal performance you will see a bullet failure. If the bullet has a bit of yaw or wobble on impact and the nose is not point on at impact then it will not get fluid into the hollow point and expand properly. This is when you see bullets recovered that look like a banana. This can result in pretty destructive results if the bullet tumbles through on the line of travel. It can also result in a bullet that turns drastically and never gets to the vitals. It can also result in a bullet that does nothing and pencils. We learned this when we first started impact testing bullets in the beginning of marketing Hammers. We were low velocity testing to confirm proper performance at 1800 fps. Launched a bullet at 1800 fps and recovered it looking like it could be loaded again. The only distortion was engraving from the rifling. Very disappointing as this is our advertised min. We decided to increase the vel to find out the min for this particular bullet, but could not get any more powder in the case. We had another rifle of the same caliber with a larger case so we grabbed it. First launch was right at 1800 fps. I remember thinking " That was a wasted shot". Went and dug the bullet out of the media to find a perfectly deformed shank. Hmmm... The only diff was barrel twist. Lots of lightbulbs went on at that moment. Ran the stability numbers and the first built was about 1.2 sg and the second bullet was over 1.5 sg. Just because you can get a bullet to an animal accurately does not mean the bullet will perform properly. Marginally stable for ballistics can shoot extremely accurately but significantly increase the odds of failure to expand. [/QUOTE]
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Bullet RPM and performance
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