TWIST, STABILITY, INSTABILITY, TIPPING AND KEYHOLING

I was led to understand that barrels were rifled to increase accuracy; and that there was a “twist”, measured as the number of inches for one revolution of the bullet. A 10” twist barrel turns the bullet one revolution in 10 inches of forward travel. And there were two corollaries; at too slow a twist, instability happened, and at too fast a twist, accuracy suffered. Opinions varied, as they will.

Some of the twist rate formulas use bullet length and velocity in the calculations. I have 22 caliber barrels in 14”, 12” and 9” twist, in 223 and 22-250.

Reducing velocity and increasing bullet length should produce instability; tests are under way.

The .22 Nosler Varmageddon 40 grain, .665” long bullet has a Greenhill minimum twist of 11.4”. The 223 Iffland barrel has a twist of 14”; so we would expect instability.