Definition:

Barrel twist refers to the rifling in the barrel of modern rifles making a full 360-degree rotation in a given length of inches. For example, a 1:7 twist means the rifling makes one complete rotation for every seven inches of barrel. The first number will indicate how many rotations the bullet will make, the second number will indicate how much barrel length it will take to complete the indicated number of rotations.

Here is a formula for calculating twist rate:

Here are the usual twist rates for most of the popular rifle calibers:

.17 HMR = 1 in 9"
.22 Long Rifle = 1 in 16"
.222 Remington = 1 in 14"
.223 Remington = 1 in 12" (varmint rifle)
.223 Rem./5.56mm NATO = 1 in 7" (AR-15)
.22-250 Remington = 1 in 14"
.243 Winchester = 1 in 10"
6mm Remington = 1 in 9"
.240 Wby. Mag. = 1 in 10"
.25-06 Remington = 1 in 10"
.257 Wby. Mag. = 1 in 10"
6.5x55 Swedish Mauser = 1 in 7.5"
.260 Remington = 1 in 9"
.264 Win. Mag. = 1 in 9"
.270 Winchester = 1 in 10"
.270 WSM = 1 in 10"
.270 Wby. Mag. = 1 in 10"
7x57 Mauser = 1 in 9"
7mm-08 Remington = 1 in 9.25"
.280 Remington = 1 in 9.25"
7mm WSM = 1 in 9.5"
7mm Rem. Mag. = 1 in 9.25"
7mm Wby. Mag. = 1 in 10"
.30 Carbine = 1 in 16"
.30-30 Winchester = 1 in 12"
.308 Winchester = 1 in 12"
.30-06 Springfield = 1 in 10"
.300 WSM = 1 in 10"
.300 Win. Mag. = 1 in 10"
.300 Wby. Mag. = 1 in 10"
7.62x39 Soviet = 1 in 10" (Ruger)
.303 British = 1 in 10"
.32 Win. Spec. = 1 in 16"
8x57 JS Mauser = 1 in 9.25"
.338 Win. Mag. = 1 in 10"
.340 Wby. Mag. = 1 in 10"
.357 Mag. = 1 in 16"
.35 Remington = 1 in 16"
.35 Whelen = 1 in 16"
.350 Rem. Mag. = 1 in 16"
.375 H&H Mag. = 1 in 12"
.378 Wby. Mag. = 1 in 12"
.416 Rem. Mag. = 1 in 14"
.416 Wby. Mag. = 1 in 14"
.44 Rem. Mag. = 1 in 20"
.444 Marlin = 1 in 20"
.45-70 Govt. (Marlin and Ruger rifles) = 1 in 20"
.450 Marlin = 1 in 20"
.458 Win. Mag. = 1 in 14"
.460 Wby. Mag. = 1 in 16"

Conclusion:

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. Fortunately, most bullets tend to be somewhat forgiving and deliver good accuracy even when the numbers say they shouldn't, which is why gun-makers can usually get away with offering only one particular rate of twist for each caliber they manufacture.