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03lover
03-03-2010, 01:56 AM
I read in one of the forums a person had an example of calculating boolit RPM using the velocity of the boolit and the twist rate of the bore. There has to be more than those two numbers needed to come up with the boolit RPM.

Does anyone know the formula for calculating Boolit RPM?
Rock
03-03-2010, 02:42 AM
Bullet RPM = MV X 720/Twist Rate (in inches)

http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/
03lover
03-03-2010, 02:48 AM
Thanks.

That is what I was looking for.
Larry Gibson
03-03-2010, 12:23 PM
Bullet RPM = MV X 720/Twist Rate (in inches)

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

That formula isn't quite correct. It will get you close enough but it is not exact as it uses the constant of 720. The formula ballisticians use is;

R=(12/T)*V

T = twist of rifling in inches
V = velocity in feet per second
R = Revolutions per second. To get RPM (Revolutions Per Minute) simple multiply the answer by 60 or;

RPM = (12/T)*Vx60

That will get you the correct RPM. However in reality either formula is close enough. I usually round off the RPM figures to the nearest 100 anyway. This is because the velocity will be an average and have a +/- extreme spread thus in reality RPM will have an ES also.

Larry Gibson
yondering
03-03-2010, 01:49 PM
That formula isn't quite correct. It will get you close enough but it is not exact as it uses the constant of 720.

Uh, yes, it is correct.
Larry, your formula is exactly the same as what Rock posted. 12x60=720
[smilie=s:
Larry Gibson
03-03-2010, 04:28 PM
Yonderling

Using that constant gives corret RPM with a 12" twist. It is not correct when other twists are used.

Larry Gibson
chboats
03-03-2010, 04:49 PM
Larry
The formula is correct. the 12 is not the twist rate but the number of inches per foot. 720 is just combining the inches per foot with 60 seconds per minute.
Carl
Larry Gibson
03-04-2010, 09:43 PM
I stand corrected. I am getting dyslexic Iguess, the formula Rock posted was not the one I was thinking of. Both formulas get the same answer. Thanks for pointing that out to me.

Larry Gibson