math formula


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moooose102
December 15, 2008, 10:05 PM
does anyone have a formula to find out how many rpm a bullet would spin out of a barrel? i was thinking of this in regaurds to bullet weight, and how many rpm it takes to stabilize it.

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BrandonBowers
December 15, 2008, 10:09 PM
Don't know the formulas but here's a couple of web sites that might help your investigations.



This one has the exact formula

http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/



Other discussion page on it.

http://yarchive.net/gun/ammo/bullet_spin_rates.html

hankdatank1362
December 16, 2008, 12:47 AM
Seems simple. Take distance traveled over a one second flight time. Say, 1/2 mile. 2640 feet.

Factor in twist rate. For sake of ease, let's assume 1/12" twist, so in one half mile's time (one minute) the bullet has spun 2640 times.

Multiply that by 60 (seconds per minute.) = 158,400

158,400 RPM is your answer.

Canuck-IL
December 16, 2008, 05:55 AM
MV X 720/Twist Rate = RPM
/Bryan

moooose102
December 21, 2008, 09:13 AM
Canuuck-IL, thanks! the rpm difference between twist rates is AMAZING! !

MachIVshooter
December 21, 2008, 10:34 AM
Canuuck-IL, thanks! the rpm difference between twist rates is AMAZING! !

And aslo illustrates why hyper velocity cartridges like the .17 Rem., .220 Swift and others approaching or exceeding 4,000 FPS must have a slower twist. There comes a point when the centrifugul force exceeds the integrity of the bullet and they come apart.

moooose102
December 21, 2008, 05:32 PM
i wonder ow many rpm it would take to make a jacketed rifle bullet come apart? i would love to see that on a slow motion camera!

Dave P
December 21, 2008, 05:35 PM
Bullet blow up happened today to one of our AR shooters at the 600 line during a LEG match. Oops - one way to lose 10 points!

Mal H
December 21, 2008, 09:23 PM
i wonder how many rpm it would take to make a jacketed rifle bullet come apart?
Several years ago I wanted to see what would happen myself. I had a bunch of Speer 50 gr "TNT" bullets which I would normally load fairly lightly in 22-250 rounds. I loaded up a few that would approach 3750 fps and a little over 190,000 RPM's and shot them at a 50 yd target.

Speer recommended not shooting TNT's faster than 3400 fps. They were right. (What are the odds?!) Pieces of the bullets did make it to the targets, but it looked more like I had shot it with #12 snake shot instead of a jacketed bullet. Impossible to tell how far the bullet got before breaking up, but it probably wasn't very far.

4sooth
December 21, 2008, 09:28 PM
A good rule of thumb for a 1 in 10 twist is about 50000 rpm for every 1000 feet per second.

Mal H
December 21, 2008, 11:56 PM
That's not a very accurate rule of thumb since it is off by about 1/3 which is significant when talking about such high rotation rates. (The actual RPM's for the given twist rate is about 72,000 RPM's per 1000 fps.)

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