|
Rifling Twist Rate By Chuck Hawks Inside of a rifle barrel there are spiral grooves, called rifling. These are intended to spin the bullet to keep it stable (point on), without wobbling or tumbling, during its flight to the target. The tighter the spiral grooves, the faster the bullet spins. The tightness of the spiral is called the "twist rate." The rate of twist is expressed as one turn in so many inches (i.e. 1 in 10" or 1:10). The caliber, length, shape and velocity of a bullet determine its optimum twist rate. The standard twist for a rifle barrel is designed to stabilize the range of bullets and velocities normally associates with that particular cartridge out to very long range. Spinning a bullet markedly too slow or too fast is detrimental to accuracy. 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 bullets require faster twist rates than shorter bullets. A faster twist increases pressure, barrel wear and also the strain on the bullet jacket, which can actually come apart if spun too fast. This particularly applies to frangible varmint bullets, which have very thin jackets, fired at high velocity in very fast twist barrels, such as the 1 in 7" twist barrels supplied on many .223/5.56mn AR-15 type rifles. 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, introduced at the same time as the .243 Winchester. When first introduced, barrels for the .244 were made with a 1:12" 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. .243 Winchester barrels, on the other hand, were rifled 1:10" to stabilize bullets as heavy as 105 grains. It turned out, most customers wanted to use their new .24 caliber rifles for hunting medium size big game, with 100 grain bullets. Needless to say, .244 sales languished while .243 Winchester sales soared. 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. Another peculiar rifling twist story involves the .223 Remington, called the 5.56mm NATO by the military. This cartridge was designed specifically for use with 55 grain spitzer bullets and its twist rate was originally specified as 1:14". This was later tightened to 1:12" for long range shooting, which works very well with bullets weighing 40-60 grains, the usual .22 centerfire bullet weight range. Years later, the military decided they needed more penetration, so they increased the normal service cartridge bullet weight to 62 grains and some military cartridges use bullets at heavy as 77 grains. Meanwhile, some hunters could not resist trying their .223 rifles on small deer and they also used heavier bullets, up to about 70 grains. The standard 1:12" twist could not properly stabilize these heavy bullets, so the military went to a 1:7" twist, designed to stabilize bullets weighing 62-77 grains. Most manufacturers of AR-15 type civilian rifles followed suit. Unfortunately, 1:7" is too fast for frangible varmint bullets, so now we have two "standard" twist rates for .223 rifles: 1:12" for varmint rifles and 1:7" for military type rifles. The latter are often, but not always, stamped "5.56mm." 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. 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, 2016 by Chuck Hawks. All rights reserved.
|
|
