Determining Twist Rate
The twist rate of the barrel on any rifle has a lot to do with bullet stabilization. Twist rate is defined as the distance in inches of barrel that the rifling takes to make one complete revolution. An example would be a 1:10 twist rate. A 1:10 barrel has rifling that makes one complete revolution in 10 inches of barrel length. Rifle manufacturers usually publish twist rates for their standard rifle offerings and custom barrels are always ordered by caliber, contour, and twist rate. Be sure to record the twist rates in your records or logbooks. If you are having a custom barrel chambered you can ask the gunsmith to mark the barrel with the twist rate.
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.
Most of you know that the twist of the rifling in the barrel is what puts spin on the bullet. As a bullet is pushed down the barrel and compressed into the rifling, the bullet follows the path or twist of the rifling. As it accelerates and leaves the barrel, the bullet is spinning at a tremendous rate. The combination of velocity and bullet spin is what stabilizes the bullet. A very popular example of twist rate affecting bullet selection is the use of bullets with rifles chambered in 223 Remington. Standard rifles chambered in 223 Remington will normally have a twist rate around 1:12. These rifles shoot very well when using ammunition loaded with 40 grain to 55 grain bullets. To use the heavier bullets such as the 69 to 80 grain bullets that are used in competition, one needs to have a barrel with a twist rate of at least 1:10 to stabilize the 69 grain bullets and a 1:8 to stabilize the 80 grain bullets. There are some 90 grain match bullets available now that need a 1:7 twist barrel to stabilize them.
Finding the twist rate for your barrel will help you in selecting appropriate weight bullets for your firearm. Remember, the general rule is that the faster the twist rate, the heavier the bullet you will be able to stabilize.