Rifling in a barrel is spiraling grooves cut into the bore that twist at a rate along the length of the barrel. These rates of twist impart a degree of spin on a bullet as it is fired down the bore, stabilizing the bullet and improving accuracy. Projectile weight primarily determines what twist rate is required to properly stabilize the bullet. Over- or under-stabilizing a particular bullet will result in a flight path that are undesirable and inaccurate. Imagine a football flying with a perfect, tight spiral as opposed to wobbling off-axis. Thrown with the same velocity, one ball will be much more accurate and fly further. Twist rate is read as a ratio, where the first number represents one complete rotation of the bullet and the second number represents the distance the bullet must travel down the barrel, in inches, to achieve one complete rotation. For example, a common twist rate is 1:8. For these barrels, the bullet achieves one full rotation in 8-inches of barrel length. Let's cover how to select the proper twist rate for your application.: 1) .223/5.56: 55 - 77 gr / 1:9; 1:7 1:8; 2) .224 Valkyrie: 75 gr. Plus / 1:7; 1:8; 3) 6.5mm: 120 - 140 gr. / 1:7; 1:8 1:9; 4)300BLK: 110 - 125 gr. / 1:07; 5) 300BLK (Subsonic): 200 gr. / Plus 1:08; 6)7.62x39: 122 - 124 gr. / 1:10, 1:9, 1:8; 7) .458 SOCOM: 500 gr. Plus / 1:14; 1:18; 8) .450 Bushmaster: 450 gr. / 1:24; 1:16.