A “barrel twist rate” is referring to the rate of spin in the rifle barrel, indicated as inches per turn. For example, a 1:10″ twist rate means that the rifling will spin the bullet one revolution in ten inches. The lower the number, the faster the twist rate of the barrel. For example, a 1:9″ twist is a FASTER twist rate than a 1:10″ twist since the bullet makes a full rotation in less barrel length.
Different bullets will require different twist rates in order to stabilize and get the most accurate and precise downrange results. If you have too slow of a twist rate your BC can be compromised, and in extreme situations, the bullet may even tumble mid-flight. This can be seen as a “keyhole” on the target, which means that you’ll see an oval shape instead of a circle.