Twist Rate

A friend of mine just jumped into the AR-15 game and wasn’t clear on how to select the optimum twist rate for the various bullets he wanted to use. He had been told that the faster twist rates (1:7-inch) require a heavier bullet (75 grains) with longer bearing surfaces, and that a slower twist (1:9- or 1:10-inch) will shoot the lighter, shorter 50-gr., 52-gr., and 55gr. bullets. I explained that while his combinations of bullet weights and rifling twists are correct for the .223 Rem. cartridge, it’s the bullet that requires the right twist, and not necessarily the twist that requires the right bullet.

The most common formula for estimating twist/bullet compatibility is known as the “Greenhill Formula.” That formula, T*L=150 (where T is the twist rate in calibers and L is the length of the bullet in calibers) applies specifically to lead bullets, but can also be used for jacketed lead bullets. Its simplicity does not do gyroscopic stability or Sir George Greenhill’s work justice.

Bullets commonly used in .223-chambered rifles come in a broad range of lengths depending on their weight, base design, and whether they’re jacketed lead or solid alloy. The longer the bullet, the tighter the rifling twist should be to stabilize it.

Bullets commonly used in .223-chambered rifles come in a broad range of lengths depending on their weight, base design, and whether they’re jacketed lead or solid alloy. The longer the bullet, the tighter the rifling twist should be to stabilize it.

As indicated by the formula, it’s the bullet’s length, not bearing surface or weight that’s the key to stability. Long, skinny bullets have minimal moment of inertia about their longitudinal axis and maximum moment of inertia about an axis through their center of gravity perpendicular to their longitudinal axis requiring a faster twist to prevent them from turning over in flight. While many will disagree, I don’t think that you can “over stabilize” a bullet to its detriment by using too fast a twist unless using a very fragile bullet at a very high velocity. In that case, the centrifugal force may exceed the yield strength of the bullet causing it to fly apart in flight and in the .223, it’s unlikely you’ll reach that high a velocity.

The take-away: the longer the bullet you intend to use, the tighter the twist needs to be.

 

Scott Mayer

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Scott Mayer

Scott Mayer

Writer at John1911
Mayer began his outdoor industry career in 1993 on the NRA Technical Staff where he became American Rifleman magazine first Shooting Editor. Mayer left NRA and entered the business end of publishing in 2003 as Advertising Account Executive for Safari Club International SAFARI Magazine and Safari Times newspaper. In 2006, Mayer was named Publisher of Shooting Times magazine where he was also tasked with launching and leading Personal Defense TV, the first television show of its kind.

In 2008, Mayer returned to the editorial side of publishing, this time in the digital field, as Editorial Director for Guns & Ammo, Shooting Times, Handguns and Rifleshooter online magazines. After a brief stint in 2011 as the Digital Media Director for an ABC TV affiliate, Mayer returned to the outdoors industry and Safari Club International where he is currently Assistant Publisher and Multi-Media Communications Editor.
Scott Mayer

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