Friday, December 18, 2009

How Rifle Twist Works

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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 a lot on the weight and length of the bullet. As you might imagine, heavier bullets require a tighter twist in order to spin them up enough to stabilize them. The length of the bullet is important, because a longer bullet has more surface area for the rifling to engage. That additional surface area gives the rifling more grip on the bullet and consequently enables it to spin it up more easily.

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 1x7 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 also 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 9 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. 1x9 and 1x10 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 1x12 boasts excellent accuracy on standard and lighter projectiles in the 40-52 grain range. Older M16 rifles were manufactured with the 1x12 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 1x12 twist ratio. If you are buying a varmint rifle chambered in .223, chances are it will sport a 1x12 Twist Ratio.

1 comment:

  1. I'm amazed that in talking about twist rate no one ever mentions barrel length. A 24 inch 1x12 will stabilize (up to a) a 62gr projectile every bit as good as an 18 inch 1x9. Bullet weight has nothing to do with the equation. Bullet LENGTH and barrel-length x twist-rate means everything. Also a 62gr flat base projectile will be more readily stabilized than 62gr boat tail in any barrel because it is shorter... however it retains it's stability for a shorter length of time (not distance) compared to the BT.