A Guide to Barrel Rifling and How It Works

When you bought your DPMS 308 AR 15, you saw the twist rate was 1:10. What does that mean? How does that affect your shooting? Would it be different in a 556 AR? Barrel rifling greatly affects how a bullet travels through the air. Here is what you need to know about barrel rifling, twist rate, and what guns have rifling.

What Is Barrel Rifling?

If you look down the barrel of a rifle or pistol—being sure it is unloaded—you will see grooves in the barrel. This is the barrel rifling. It causes a bullet to spin as it travels down the barrel. When in flight, this gyroscopic stability improves the bullet’s accuracy. It helps the bullet fly smoothly. Think of how a quarterback throws a football, and the football flies in a tight spiral. It is the same concept with a bullet. Without the spiral, the bullet will wobble and tumble.

Two Types of Rifling

There are two types of rifling. The standard type has a series of alternating low and high spots, called grooves and lands. Polygonal rifling uses less pronounced hills and valleys, which are in the shape of a polygon. You are not likely to find polygonal rifling on your LWRC AR 15, though, as they are mostly used in pistols. Rifles usually have standard rifling. The tolerances in a barrel are very small, to the point where a bullet stuck in the barrel cannot be pushed out by hand. The bullet is instead deformed slightly as it scrapes along the grooves.

Twist Rates and Why They Are Important

The twist rate is essentially how long a bullet takes to make a full twist through the barrel. This depends on how tight the spiral of grooves is. Using the right twist rate, or amount of spin, on the right bullet means the bullet travels an optimal distance at maximum accuracy. Too much or too little spin, and the bullet can tumble. This means it is crucial to pair a twist rate with the type of ammo you are using. Faster and slower twists are needed to stabilize bullets with different weights, sizes, and speeds. Twist rate is shown, often stamped on the barrel, as one :X, where “x” is the number of inches it takes for one full revolution of the bullet. A 1:8 twist, not uncommon for 556, is faster than a 1:12, which you might find in a 308. Heavier bullets tend to need faster spin rates.

Do Shotguns Have Rifling?

Handguns and rifles have rifling, but not shotguns. Shotgun shells do not need rifling, as it shoots pellets. Instead, chokes are used to control the spread of the pellets. That is why you are not likely to find rifling on your Benelli M4. Instead, shotguns are smoothbore. The exception is a cylinder bore, choke, or even rifled barrel meant for shotgun slugs, but these are not as common as smoothbore shotguns.

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