Term: Twist Rate
Relates to: Barrels, Rifling
Application(s) of Use: Usually rifle shooting, where multiple bullet weights are available
Definition: A twist rate of a barrel refers to the distance it takes the rifling in the barrel to make a complete revolution. The twist rate is usually provided in inches. For example, 1/7 (or 1×7 or 1-7, all are acceptable) means one complete rotation of rifling takes seven inches of barrel length. So a 1/12 barrel has a slower twist rate than a 1/9.
Why it Matters: Pairing the wrong bullet with the wrong twist rate can lead to issues both with accuracy and stabilization of the projectile, especially at longer ranges. If you want consistency, and you should, you should pay attention to the twist rate of your rifle and use projectiles appropriate for it.
Into the Weeds: Speaking in general terms, longer projectiles work better in faster twist guns. You may have heard people talking about proper grain weights of a projectile for a given twist rate, but that’s not quite right. Length, not weight, is the main issue. If the bullets you’re talking about are constructed of copper and lead (most are), then increasing grain weight de facto increases the projectile length. However, some extremely heavy bullets have been made in the past out of unconventional materials such as tungsten, which allowed for stability even with the higher grain weight because overall project length itself was not greatly effected.
Usually you’ll find the twist rate stamped on the barrel, but in the case of an unmarked barrel, all is not lost. Here is an excellent overview from Accurate Shooter:
If are unsure of the twist rate of the barrel, you can measure it yourself in a couple of minutes. You need a good cleaning rod with a rotating handle and a jag with a fairly tight fitting patch. Utilize a rod guide if you are accessing the barrel through the breech or a muzzle guide if you are going to come in from the muzzle end. Make sure the rod rotates freely in the handle under load. Start the patch into the barrel for a few inches and then stop. Put a piece of tape at the back of the rod by the handle (like a flag) or mark the rod in some way. Measure how much of the rod is still protruding from the rod guide. You can either measure from the rod guide or muzzle guide back to the flag or to a spot on the handle. Next, continue to push the rod in until the mark or tape flag has made one complete revolution. Re-measure the amount of rod that is left sticking out of the barrel. Use the same reference marks as you did on the first measurement. Next, subtract this measurement from the first measurement. This number is the twist rate. For example, if the rod has 24 inches remaining at the start and 16 inches remain after making one revolution, you have 8 inches of travel, thus a 1:8 twist barrel.
Environmental factors such as altitude and ambient temperature also have a direct effect on projectile stability. For example, a heavy 77gr 5.56 projectile may be just fine when fired out of a 1/9 barrel at high altitude or during warm weather, but be unstable when fired at sea level in cold weather. There is no “ultimate” overall twist rate since caliber, velocity, and projectile construction all play a part. JBM has a bullet stability calculator that takes environmental factors into account, but bear in mind that sometimes twist rate itself is rounded by manufacturers (because labeling a barrel with 1/8.36575 just isn’t very practical).
As far as practical and modern uses, it’s better to use a shorter projectile in a faster twist barrel than a longer projectile in a slower twist barrel. For the AR-15 owners out there, it means that while far from ideal, a 45gr bullet in a 1/7 barrel is better than a 77gr bullet in a 1/12 barrel–but it’s absolutely best if you pair the two properly to begin with.
In Summary, take twist rate into consideration when purchasing a firearm, and always try to use a projectile suitable for your twist rate. Generally it’s much easier to start with the barrel you want than have to purchase specialized ammunition.