BULLETS AND RPM
Bullets in flight are spun by the rifling in the gun barrel. The rate of this bullet spinning is commonly measured in revolutions per minute, RPM.
MV = bullet muzzle velocity in feet per second.
T = twist, the number of inches for one bullet revolution in the barrel.
* = multiplied by.
RPM = (720 * MV)/T
MV = (RPM * T)/ 720
T = (720 * MV) / RPM
To increase RPM, either MV must be increased, the T number must be reduced = made faster, or both.
To decrease RPM, either MV must be decreased, the T number must be increased = made slower, or both.
Any RPM defines an array of T and MV.
Using the Greenhill formula we can solve for bullet length, and using a survey ratio, solve for bullet weight.
This table shows, for 120000 and 140000 RPM, an array of MV, T, bullet length “, and bullet weight grains; each of which solve for the RPM.
For example, .308 bullets at 120000 RPM, at 2000 fps, T = 12”, bullet length = 1.19”, and bullet weight = 207 grains.
This is one of the 16 solutions shown; there are essentially an infinite number of solutions.
These solutions may be divided into “feasible” and “unfeasible”, with the borders subjective.
For example: A .308” bullet, 2.37” long, weighing 415 grains, in a 6” twist barrel, at 1000 fps, spins at 120000 RPM. I would consider this solution to be in the unfeasible set.
This table contains a lot of feasible solutions; maybe the most for a 20000 RPM span. If so, increasing or decreasing RPM would diminish the number of feasible solutions, and probably accuracy.
Feasibility varies with RPM. The 120000 to 140000 span has a lot of feasible solutions; but see the .457” bullet diameter table.
There are not many feasible solutions on this table. A 502 grain bullet at 4000 fps? A 6.09” long bullet, 2344 grains, at 1000 fps? Perhaps there are none.
Bullet RPM, MV and Twist are mathematically related, if we know MV and Twist, we know RPM. RPM is a representation of MV and Twist. Feasible solutions are those that have been shown to work, to give at least reasonable results.