How Rifle Twist Works

Story courtesy of Cheaper Than Dirt's "The Shooter's Log," at www.CheaperThanDirt.com/blog

Just reading the title, you might think this would be a very short post. Everybody knows that rifle twist works by spinning the bullet so that it is stable as it flies through the air. Naturally, there’s a bit more to it than that.

Anyone that spends any amount of time at the rifle range or hunting lease will inevitably find himself within earshot of two people discussing barrel twist. Twist as discussed here, refers to the rifling in the barrel of modern rifles making a full 360 degree turn in a given length of inches. As an example, a 1 in 7 Twist means the rifling makes one 360 degree turn for every seven inches of barrel. By the same token, 1 in 9 twist means the rifling comes a full 360 degrees in 9 inches. So, the lower the number of inches, the tighter the twist of the rifling.

The amount of twist you need in a given rifle depends primarily on the length of the bullet. Longer bullets require more spin to maintain gyroscopic stability. Think of it like the difference between throwing a baseball and throwing a football: the football requires more axial spin to remain stable in flight than does a baseball. As you might imagine, this is why heavier bullets for a given caliber require a tighter twist. For convenience, most people refer to bullet weight when discussing barrel twist since when discussing any given caliber of ammunition a heavier bullet will almost always be longer than a lighter bullet.

Many firearms are manufactured with a variety of rifling twists available. For the popular AR-15 platform, several different twists are currently produced. Not all ammo shoots well in all twist ratios. A barrel with a 1×7 Twist tends to be too tight for most lighter, more commonly fired ammunition, but is perfect for heavier bullets in the 69-80 grain range. Firing a light weight 55 grain bullet through a tight twist barrel can “overspin” the bullet and result in a loss of accuracy as the rapidly spinning bullet curves through the air, not unlike a curve ball. Overspinning the bullet can even cause some thin jacketed hollow point projectiles to fly apart from the centrifugal forces of the spin.

Standard military issue M16 and M4 rifles, and their AR counterparts, are commonly found with 1 in 7 twist barrels. Originally designed for the military’s use of SS109 (the official NATO name of 5.56mm, or .223), military testing concluded this twist ratio is actually superior for this steel core bullet. 1×9 and 1×10 twist ratios are sort of the “middle of the road” for .223 projectiles, and these are the most common. We suggest our Lake City military XM193 ammunition for this barrel.

On the lighter side of things, a slower 1×12 boasts excellent accuracy on standard and lighter projectiles in the 40-52 grain range. Older M16 rifles were manufactured with the 1×12 Twist ratio. Our item number ARR-115 offers a conversion upper for your AR that takes less than a minute to install, and the barrel has a desirable 1×12 twist ratio. If you are buying a varmint rifle chambered in .223, chances are it will sport a 1×12 Twist Ratio.

Posted on 1/3/2011