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I have some 55 grain Speer TnT bullets I bought in the mid 90's. The statement from Speer was not to exceed 3400fps. The failure of the bullet is based not on its linear velocity but on its rotational velocity.

Most barrels for 223 and 22-250 were twisted 1/12 to 1/14. My Windham Varmint Exterminator is twisted 1/8. 1/8 is 33% faster than 1/12. So theoretically I should only be able to fire that bullet at 66% of the maximum of 3400 fps to have the same rotational velocity. That would be 2240 fps by what I figure.


Am I wrong?


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I don't think their is a direct ratio between velocity and rotational speed. I have no idea how to compute the answer. I'm pretty good at numbers, but those fancy equations with the letters and symbols in them just make my head hurt! Guess that's why I went to law school. grin

Maybe Mathman will stop in later and give a good answer and explanation. I'm kind of curious myself.


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no matter what your linear velocity the rotational velocity should be the same
rotational should be 1 rev per 8 inches of travel no matter what the linear speed is. if the bullet can't stand up to the 1 rotation in 8 inches it will fail at 1000 fps or 3000fps just my ruminations.
i fired some 55's from a 30-06 using sabots at ground squirrels with a chrono speed or 4200 fps . they worked. stepped up to 4300 and there was a puff out of the muzzle. and my chrono got peppered.
yes they were speer tnt's ymmv


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3400 fps divided by 12 then multiplied by 8 gives 2267 fps (rounded off) so the rpm figures match.

Whether or not the entirety of bullet failure is attributable to rpm I don't know.

Where people get off track is thinking about the relationship between rpm and fps after the bullet leaves the barrel. On the trip downrange the bullet loses linear velocity faster than it loses angular velocity.

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Originally Posted by deerstalker
no matter what your linear velocity the rotational velocity should be the same
rotational should be 1 rev per 8 inches of travel no matter what the linear speed is. if the bullet can't stand up to the 1 rotation in 8 inches it will fail at 1000 fps or 3000fps just my ruminations.
i fired some 55's from a 30-06 using sabots at ground squirrels with a chrono speed or 4200 fps . they worked. stepped up to 4300 and there was a puff out of the muzzle. and my chrono got peppered.
yes they were speer tnt's ymmv


Nope. That relationship changes once the bullet leaves the barrel.

If you mean within the barrel then yes it stays one turn per eight inches. But how quickly the eight inches goes by, and hence how quickly the one turn goes by, does depend on the linear speed.

Last edited by mathman; 05/26/17.
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If the bullet leaves a 1/12 barrel at 3400 FPS it is spinning at 3400 RPS. If out of a 1/8 barrel it would be 5100 RPS. It would seem to me that it doesn't matter if the rotation slows down, or how much, because the maximum stress on the bullet would be as soon as it leaves the support of the barrel.

Last edited by Armednfree; 05/26/17.

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Try some, see what they do.

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Originally Posted by mathman
Originally Posted by deerstalker
no matter what your linear velocity the rotational velocity should be the same
rotational should be 1 rev per 8 inches of travel no matter what the linear speed is. if the bullet can't stand up to the 1 rotation in 8 inches it will fail at 1000 fps or 3000fps just my ruminations.
i fired some 55's from a 30-06 using sabots at ground squirrels with a chrono speed or 4200 fps . they worked. stepped up to 4300 and there was a puff out of the muzzle. and my chrono got peppered.
yes they were speer tnt's ymmv


Nope. That relationship changes once the bullet leaves the barrel.

If you mean within the barrel then yes it stays one turn per eight inches. But how quickly the eight inches goes by, and hence how quickly the one turn goes by, does depend on the linear speed.

blush pardon me as i am old and it has been centuries since i took physics! linear v will increase rv .
had my coffee !


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Misplaced it, but years ago, I called Hornady on one of their bullets, the SPSXs, that was giving just a puff like your breathe on a cold day, when it was 90 degrees outside.

they gave me the formula for figuring out the RPMs the bullets were turning out of the Muzzle.

said the SPSX was design for about 180,000 RPMs...

my load with a one in 7 twist rifle, they were turning like 320,000 RPMs...

disintegrating about 6 inches outside the muzzle.


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I tried to shoot some of the bulk bullets in my quick twist 22-250,55 grains as well as others.

Most never hit the target at 100 yards and those that did were sideways.

It was a hoot seeing the puff of grey smoke when they shed the jacket.

The only bullet that would handle the speed was the nosler 55 grain b-tip.

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Good luck with those 55grainers. I had some 50gr. TNTs also from the early 90s. I tried them in my 1 in 9 twist Savage with a 26" heavy barrel and had to keep them in the 2900-3000fpr range just to get them to hit a 100yd. target. I gave up and switched to the 50gr. VMAX. Thousands of CO and WY prarie dogs have been turned into red mist from those vmax bullets. I think the TNTs will work if you had a slower twist but not in a 1 in 8.

Just my experience

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Figuring out the RPM's of a bullet is actually pretty easy, it's just a matter of converting units.

Say your bullet is doing 3500 feet per second out of a 1 in 8 inch twist barrel and you want to convert to revolutions per minute.

Multiply 3500 feet per second times 60 because there are 60 seconds in one minute. That equals 210,000 feet per minute the bullet is travelling.

Now you've got to figure out how many revolutions the bullet makes in a foot. Since it makes one revolution in 8 inches divide 12 by 8 since there are 12 inches in 1 foot. That equals 1.5 revolutions per foot.

Now since it travels 210,000 ft per minute and it revolves 1.5 times in every foot of travel multiply 210,000 times 1.5. That equals 315,000 RPM that bullet is turning.

So the formula to figure out the RPM of a bullet is: Muzzle velocity X 60 X 12/twist rate = bullet rpm


The higher the rpm the higher the centrifugal force that makes the bullet want to fly apart. Bullet construction is critical to surviving high velocities out of fast twist barrels. I've shot 40 gr. ballistic tips out of my 1-8" twist 22-250 at 4200 fps and they held together just fine at the 378,000 RPM they're turning. If you shoot a lightly constructed hollow point like the speer TNT at those rpm's and you'll probably just see a puff in front of the barrel. It doesn't mean the speer is a bad bullet, it's just not designed for that.

The plastic tipped bullets seem to be able to withstand these extreme rpm's a lot better than hollow point or exposed lead tip bullets.

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I don't see the need to change the terms to RPM, RPS works fine.


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RPM is the common unit for rotational speed and is easy for people to visualize. If you call the techs at most bullet companies they can tell you what rpm the bullet is designed to handle. It helps to be speaking in common terms in discussions. If I ask someone the temperature I'd rather they give me Fahrenheit than kelvin.

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Am I wrong?


Yes, ...but your spotter won't be,... every now and then :

"Gray mist, puff of smoke, no impact recorded."

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