Projectile Spin Rifling RPM Calculator

Bullet spin: RPM = v · 720 / twist (in/s units mix)

Helical grooves cut into a rifled barrel spin the bullet, which keeps it stable in flight the same way a spinning top stays upright. When you see a twist rate written as "1:10", that means the rifling makes one full turn for every 10 inches the bullet travels down the bore. To get the rotational speed, use RPM = (v_fps · 12 / twist_in) · 60, or the metric form RPM = v_m/s · 39.37 · 60 / twist_in. Take a .308 Winchester with a 1:10 twist leaving the muzzle at 800 m/s: that works out to about 800·39.37·60 / 10 ≈ 189,000 RPM. Spin it too slowly and the bullet starts to tumble; spin it too hard and the jacket can let go.

Applications

The Greenhill formula (twist = 150·d²/L for a typical lead core, in inches) is the classic way to pair a twist rate with bullet length, and the longer the projectile, the faster the twist it wants. Specialty rounds push you off the standard barrels entirely. Subsonic and very-low-drag loads, for instance, often call for 1:7 on heavy 77 gr 5.56 or 1:8 on a 175 gr .308 ELR. Whether you're hunting or punching paper at the range matters too. On the manufacturing side, button rifling, cut rifling and hammer forging all hold tight tolerances on the twist they produce.

FAQ

What is the stability factor (Sg)? It's a dimensionless number, usually above 1.4, that Miller's formula produces from the twist, velocity, bullet dimensions and air density. The point of it is to tell you whether the bullet will actually fly nose-first.

Does twist affect velocity? Only a little. A faster twist adds some friction in the bore and shaves a few fps off the muzzle velocity, but that's usually lost in the noise next to the variation between loads.

Why do some bullets disintegrate in flight? Thin-jacketed varmint bullets that are spun too hard can push past the jacket's tensile limit above 300,000 RPM, and they come apart not long after they clear the muzzle.