Firearms Stack Exchange Archive

What does the “twist rate” of a barrel mean?

I’m looking to purchase a new AR15 and have come across a term I’m unfamiliar with: twist. I’ve been a handgun user for a few years now, but would like to start learning more about rifles. Can someone explain what this means and why I should pay attention to it?

Answer 282

The best all-around twist rates for AR-15 rifles is 1 full rotation of the bullet per 7 inches of barrel length, or 1:7 (pronounced “one-in-seven”). This twist rate will stabilize all military and self defense loads, and will only have issues with bullet weights under 50gr. Bullet weights under 50gr are nearly exclusive to varmint hunting. A 1:9 barrel will have trouble with loads much heavier than the mid-60gr range, which puts many very effective defense, long range, and hunting loads outside their ability to stabilize.

Simply put, bullet stability is determined by the rifling’s twist rate. Twist rate is dependent upon the length of the bullet and not the weight, it just happens to be that bullets of heavier weights in a given caliber and material(s) can only grow in length as they increase in weight. To know your desired twist rate, you will need to know the range of bullet weights you will be shooting. It is possible to both under- and over-stabilize a bullet.

#Bullet stability and twist rate Twist rate, as expressed in most modern weapons, is the length of barrel required to complete one full rotation of the bullet as it travels through. A 1:7 (“one-in-seven”) twist is one full rotation for every 7 inches of barrel length. To calculate the amount of twist needed to stabilize a bullet you would use a formula known as the Greenhill Formula:

The Greenhill Formula

Note that the length of the bullet is used, and not its weight. The longer the bullet becomes, the more spin it requires to stabilize. In a given caliber, a bullet can only expand in length as its weight increases, but weight is not the deciding factor.

Early AR-15 rifles, before being adopted officially by the military, had a 1:14 twist rate suitable for firing only very light bullets. As bullet weights (and lengths) increased this has reduced to the current rate of 1:7. Commercial rifles come in, commonly, 1:7, 1:8, and 1:9. When firing, for example, 55gr or 62gr bullets any of these rates will do. However, if you were to fire a 40gr varmint round through a 1:7 you would rip the copper jacketing off. Conversely, if you fired a 77gr bullet through a 1:9 you would lack the stabilization that bullet requires, and your groups would be large and erratic. At relatively close ranges you would experience bullet tumbling and other erratic behavior.

Answer 280

The twist rate is the rotations:inches of the rifling. SO a 1:9 twist is one full rotation over 9 inches. The reason it becomes important is different bullets stabilize better at different rates. You want to match the twist to the bullets you will most commonly shoot. There is a pretty decent article on it here. The general rule is fast twist for heavier bullets, slower twist for light bullets of the same caliber.

Answer 281

The “twist” of the barrel is the pitch of the rifling within the barrel. A barrel with a 1:7 twist will spin the bullet around once in 7 inches of travel. Generally speaking, a heavier bullet needs more twist. Lighter bullets can actually break apart in midair if they experience too much twist, while heavier bullets will not stabilize if they do not get enough.

Most ammunition on the AR platform will be stabilized by a 1:7 or 1:9 twist. 1:7 is the “Government Profile” barrel’s twist, and is my personal favorite.

Answer 395

Your initial question about twist rate has been answered adeptly already, so I won’t expand on that. I do think your real question is, “Which one should I buy?” and I believe the answer is not 1:7, but 1:8. As noted above a 1:7 will rip apart some older ammunition whereas a 1:8 can shoot practically anything. And, if you get a 1:8 with a Wylde chamber, you can shoot accurately any 5.56 or .223 round made.

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