Rifling Twist Rate
By Chuck Hawks
The rate of twist is expressed as one turn in so many inches (i.e. 1 in 10" or 1:10). The twist in a rifle barrel is designed to stabilize the range of bullets normally used in that particular caliber. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight and heavier bullets require a faster twist than lighter bullets of the same shape. It is undesirable to spin a bullet a great deal faster than necessary, as this can degrade accuracy. A fast twist increases pressure and also the strain on the bullet jacket.
Fortunately, the rate of twist chosen by the rifle maker is usually appropriate for the intended cartridge. Anyone ordering a new barrel for a hunting rifle will generally do well to specify the standard twist as supplied by the major rifle manufacturers for that caliber. Where there are two twist rates in common use, for example 1:10 and 1:12 for the .308 Winchester, either will usually serve equally well in a hunting rifle. Many other factors are more important to accuracy and performance than twist rate.
Once in a great while, though, a manufacturer makes a mistake. One such case involved the .244 Remington. When first introduced, barrels for this caliber were made with a 1-in-12 twist, because Remington anticipated that their new cartridge would be used primarily for varmint shooting. The 1 in 12 inch twist is ideal for best accuracy with varmint weight bullets (70-85 grains) in a high velocity .24 (6mm) caliber rifle. The heaviest spitzer bullet that a .244 with a 1 in 12 inch twist barrel could stabilize was 90 grains. The customers, however, also wanted to use their new .24 caliber rifles for hunting medium size big game, with 100 grain bullets. Needless to say, customers ignored the new .244 Rem. Remington soon saw the error of their ways and changed the rifling of their .244 barrels to 1 turn in 9 inches, but the damage was done. Sales remained so slow that eventually Remington had to discontinue the .244. The following year they reintroduced the exact same cartridge as the 6mm Rem. and produced all 6mm rifle barrels with 1 in 9 inch twist barrels, which can stabilize all .24/6mm bullets.
Here is a formula for calculating twist rate:
Here are the usual twist rates for most of the popular rifle calibers. (For a more comprehensive list, see the expanded "Rifle Barrel Twist List" on the Tables, Charts and Lists page.)
.17 HMR = 1 in 9"
Not all rifle barrels of the same caliber have the same twist rate. A fellow ordering a custom rifle may have his own ideas about twist, as may the builder. For example, some .270 Win. barrels are rifled with a 1 in 12 inches twist, some .30-06 barrels are also rifled 1 turn in 12 inches, and some .300 Magnum barrels are rifled 1 turn in 14 inches. Usually these variations make no appreciable difference. These slower twists may give slightly lower pressure, as well as very slightly better accuracy with the lighter bullets in each caliber. They will still stabilize the heavy bullets over practical hunting ranges. However, they might not be the best choice, or quite as accurate, for shooting heavy bullets at extreme range (such as 600-1000 yards).
Copyright 2012 by Chuck Hawks. All rights reserved.