Twist rate refers to the rate of spin in the rifle barrel, and is represented in inches per turn. It’s important that your barrel has an adequate twist rate to stabilize the bullets you’re shooting. A barrel that is a 1:10” twist means that the rifling will spin the bullet one revolution in 10 inches. The lower the number of the twist, the faster the twist rate or the faster the bullet will spin. For example, a 1:8 twist will spin the bullet one revolution in 8 inches, whereas a 1:10” twist will spin a bullet one revolution in 10 inches. If you were shooting a bullet in both barrels at a velocity of 2800 feet per second, the 1:8” twist barrel (252,000 RPM) will spin a bullet much faster than a 1:10” twist barrel (201,600 RPM). So a 1:10” twist is slower than a 1:8” twist, a 1:12” twist is slower than a 1:10” twist, and so on. Generally, fast twist barrels are used for longer bullets while slower twist barrels are used for shorter bullets.
If a bullet has a twist recommendation of 1:10”, it will be stable when fired from any rifle having a 1:10” or faster. So a 1:9” would work fine, but a 1:11” may not. The recommended twist rate will ensure adequate stability in all conditions, but there are some cases when the bullet may be stable from a slower twist (high altitude for example). Use the twist calculator on this page to calculate a stability factor for any bullet in your conditions to determine the actual twist requirement.