Greenhill's Formula

This uses a constant of 150. It does **not** predict the instability that sometimes occurs
with blunt, short revolver bullets.

The formula is: * maximum bullet length = diameter x diameter x 150 / twist*

The online design program's suggested twist uses a safety factor of 1.3. This gives a good
ballpark number but *it is not intended to be the final word* so don't lose a lot of sleep worrying about it.