(feet / second) * (turns / foot) * (60 seconds / minute) = rpm Just look at which units cancel out when the fractions are multiplied.
The only trick fill in is the turns per foot. An 8" twist will give 12/8 = 1.5 turns per foot.
Example for 3000 fps in a 10" twist: 3000 * 12/10 * 60 = 216,000 rpm
