Skip to main content

AR-15 Bullet Stability

AR-15 Bullet Stability

Q: I've just jumped into the AR-15 game and I don't have a clear idea of how to select the optimum bullet weight and bullet bearing length for a certain twist rate barrel. I've been told that the faster twist rates (1:7-inch) require a heavier bullet (75 grains) with longer bearing surfaces, but that a slower twist (1:9- or 1:10-inch) will shoot the lighter, shorter 50-gr.,52-gr., and 55gr. bullets. Why is this? Paul O.

A: Your combinations of bullet weights and rifling twists are correct for the .223 Rem. cartridge, though it's the bullet that requires the right twist, not the twist that requires the right bullet. The most common formula for estimating twist/bullet compatibility is known as the "Greenhill Formula." That formula, TxL=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. As indicated by the formula, it's the bullet's length, not bearing surface or weight that's the key to compatibility. 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.--sem

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended Articles

Recent Videos

Scott Rupp gets a chance to talk to Aaron Oelger about a few new products from Hodgdon and why reloading store shelves are empty.

Hodgdon Reloading

Beth Shimanski of Savage introduces their all new straight-pull rifle. With an Accu-Fit stock and left hand adjustable bolt, this rifle is a perfect choice for anyone!

Savage Impulse

RifleShooter Magazine editor Scott Rupp breaks down all the features of the Mossberg Patriot Predator rifle chambered in 6.5 PRC.

Mossberg Patriot Predator 6.5 PRC Rifle Review

Introduced in 1965 with the .444 Marlin cartridge, the Model 444 was the most powerful lever action of its day.

Marlin Model 1895 in .444 Marlin

J. Scott Rupp takes a first look at the Springfield M1A Loaded rifle chambered in the popular 6.5 Creedmoor.

Review: Springfield Armory M1A Loaded Rifle in 6.5 Creedmoor

If looking to acquire an automated powder-charge dispensing unit to speed up precision reloading, don't judge the RCBS ChargeMaster Lite powder scale and dispenser by its name; the Little Green machine packs a heavy-weight punch with speed and accuracy.

RCBS ChargeMaster Lite Review: Not 'Lite' on Ability

The new Sako Finnlight II sports an innovative stock and Cerakote metal paired with the terrific 85 action.

RS Sako Finnlight II

The Remington Model Seven is ready, willing and able to handle just about any task.

Remington Model Seven SS HS Bolt-Action Rifle Review

RifleShooter Magazine Covers Print and Tablet Versions

GET THE MAGAZINE Subscribe & Save

Digital Now Included!


Give a Gift   |   Subscriber Services


Buy Digital Single Issues

Magazine App Logo

Don't miss an issue.
Buy single digital issue for your phone or tablet.

Buy Single Digital Issue on the RifleShooter App

Other Magazines

See All Other Magazines

Special Interest Magazines

See All Special Interest Magazines

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Get the top Rifle Shooter stories delivered right to your inbox.

Phone Icon

Get Digital Access.

All RifleShooter subscribers now have digital access to their magazine content. This means you have the option to read your magazine on most popular phones and tablets.

To get started, click the link below to visit and learn how to access your digital magazine.

Get Digital Access

Not a Subscriber?
Subscribe Now

Enjoying What You're Reading?

Get a Full Year
of Guns & Ammo
& Digital Access.

Offer only for new subscribers.

Subscribe Now