If you look down the bore of a high-quality barrel with a bore scope, you won't see any copper-grabbing tool marks running across the rifling.

And then there is twist rate. The longer a bullet of a particular caliber is, the faster it must rotated in order to maintain its stability during flight. Bullet rotation is, of course, governed by rifling twist rate of the barrel and bullet velocity.

An increase in either one will increase rotational velocity, but it takes a rather substantial increase in speed to equal a small increase in rifling pitch. When, for example, a .224 caliber 90-grain MatchKing exits an AR-15 rifle barrel with a 1:6.5-inch twist (one turn of the rifling in 6.5 inches) at a velocity of 2,700 fps, it is spinning at a rate of 4,985 revolutions per second. Keep velocity the same but increase twist rate to 1:5.5 inches and rotational speed will increase to 5,891 rps. In order to increase bullet spin by that much in a 1:6.5 twist barrel, velocity would have to be increased to 3,200 fps, which isn't possible with the .223 Remington at acceptable chamber pressures.

Choosing Twist Rates
How does one choose twist rate? One way is to pick the rate the manufacturers use, although that's not always correct. Barrels for the .257 Roberts, for example, use a 1:10 twist, which worked fine for the 100-grain bullets originally popular for the caliber but sometimes doesn't match up well with 117-grain pointed bullets. Anyone who decides to buy a barrel for the .257 Roberts today is wise to choose a 1:9 twist as it will stabilize pointed bullets weighing up to 120 grains and still deliver good accuracy with those as light as 75 grains.

The correct rifling twist rate for a particular bullet can also be calculated by using a formula originated by Sir Alfred Greenhill during the late 1800s. Simply divide a constant of 150 by the length of the bullet (in calibers) and then multiply by the diameter of the bullet (in inches).

For example, using the Roberts again, the Speer 120-grain spitzer has a diameter of .257 inch and a length of 1.125 inches. Dividing that bullet's length by its diameter gives us 4.38. Divide 150 by 4.38 and multiply the result by .257 and you arrive at a rifling twist rate of 8.80 inches. Rounding off to the next highest number gives us a twist rate of 1:9 inches which, as I said before, is correct for the .257 Roberts when it is loaded with a 120-grain spitzer.

Another and perhaps the safest way to determine the correct rifling twist rate for a new barrel is to tell the maker of the barrel what cartridge it will be chambered for and which bullets you intend to shoot and let him decide (some barrel makers post this information on their websites).

Benchrest shooters choose relatively slow rifling twist rates that border on bullet instability, but they get by with it because they usually stick with a single bullet weight. When choosing a twist rate for a rifle to be used for varminting or big game hunting, I prefer more flexibility in bullet choice and tend to err on the quick side. The accompanying sidebar contains some recommendations from Shilen.