(Originally published in the April 1998   Edition of the CMP Newsletter "The First Shot")

     In its simplest guise, a 1 in 10 twist means that a projectile traveling down the barrel rotates one complete turn around its own axis in 10 inches. Thus a 1 turn in 12" twist rotates the bullet one complete turn in 12 inches, etc. The second thing you must realize is that a 1 (turn) in 10" twist is said to be a "tighter" twist than a 1 in 12", and a 1 in 12" is tighter than a 1 in 14".

     Now that we understand the terminology, what does all that have to do with anything anyway? If you are a gun designer or a reloader, it means quite a bit from a very practical standpoint. Again from a simplistic standpoint, the tighter the twist, the heavier the bullet the barrel will stabilize for any given bore diameter. For instance, a 1 in 10" twist in a .30-'06 will stabilize a 220-grain bullet, whereas a 1 in 12" twist generally speaking will not. Many folks don't realize that the original cartridge designed for the M1903 Springfield utilized a 220-grain bullet like its predecessor the .30-40 Krag. The designers at Springfield ran many experiments to determine the most accurate twist for the new cartridge. It was found that a 1 in 8" twist was the most accurate, but due to the extremely hot nitroglycerine based powders at the time coupled with the relatively mild ordnance steel, the barrels were eroded to the point of inaccuracy within 1000 rounds. Utilizing the appropriate formulas, it was predicted tat the minimum twist that would accurately stabilize the 220-grain bullet at 2300 feet per second, was 1 in 10.5". They compromised on a 1 in 10 twist, and the accuracy remained acceptable with a considerably extended barrel life. With a change to a 150 grain flat based spitzer bullet, the need for the 1 in 10 twist diminished, but the accuracy remained excellent so there was no need to change the twist. The M1 Ball cartridge adopted after WWI with its 173-grain boat-tailed bullet absolutely thrived on the old 1 in 10" twist. The M1 was designed around the M1 Ball round and the 1 in 10 twist was retained (if it ain't broke, don't fix it!).

     The design of the M14 Rifle did not anticipate a return to a 220-grain bullet, so the new design used a 1 in 12" twist. As anyone who has shot a 1 in 12" twist 7.62mm with the old M118 Ball (172 grain bullet at 2550 fps.) or the new 168 grain Sierra Bullet can tell you, the combination is superbly accurate. I have personally seen a 1 in 14" twist M14 experimental barrel display absolutely phenomenal accuracy with the same 168 grain Sierra bullet. That same barrel however would not keep the 172 grain M118 load in the black at 300 yards… we had found the so called "crossover point".

     When the M16A1 was redesigned to fire the heavier 63-grain bullet as opposed to the older 55-grain bullet, the twist was tightened to 1 in 7". Supposedly the extreme change was due to poor performance of the 63-grain bullet using "looser" twists under Arctic temperatures. Recent experiments with the extremely accurate match Service Rifles built by military armorers have indicated that under temperate conditions, a 1 in 8" twist seems to be optimum. Some are using 1 in 7.8" twists, etc., but the idea is still the same - a heavier bullet needs a tighter twist!

     What does this mean to the average shooter? Well, if you are planning on going to Alaska on a hunting trip and wish to use a .308 Winchester as an all around rifle (not recommended), you might consider buying a Ruger .308 which uses a 1 in 10 twist vice a Winchester (U.S. Repeating Arms) which uses a 1 in 12. With a 1 in 10 twist, you can handload a 220-grain bullet as an emergency anti-Grizzly device (I would personally prefer a LAW Rocket). Shooters participating in 1000 yard competition with 300 Win. Mag. or .30-'06 bolt guns or 7.62mm M1As might consider having their fowling pieces barreled with 1 in 10 twist barrels to take maximum advantage of heavy bullet loads to buck the wind at longer ranges.