a 1:9 twist is one revolution per 9 inches, 1:12 is the one per 12 inches
1:9 is a faster rate than 1:12
Heavier/ longer bullets need to be spun faster than lighter bullets to be stabilized while in flight
but if you spin a light bullet to fast, it can come apart and if you shoot a heavy bullet with to much velocity for the twist rate than it can't grip the rifling
you match the twist to the bullet weight and velocity you want to shoot.
The better you match the rate to the rond the better accuracy you will get.
get it wrong and accuracy will become very poor at long distances
But there is room in the #'s
for .308 you can shoot 155, 168 and 175 grain all out of the same twist rate, but the accuracy will fall off as the distance increases, about 600 yards for 168gr and 800 yards for 175gr, out a 20" inch barrel, increase the barrel length will increase the velocity keeping the bullet stabilized longer, so 24" barrel may get you to 1000yards with 175gr
1:9 is a faster rate than 1:12
Heavier/ longer bullets need to be spun faster than lighter bullets to be stabilized while in flight
but if you spin a light bullet to fast, it can come apart and if you shoot a heavy bullet with to much velocity for the twist rate than it can't grip the rifling
you match the twist to the bullet weight and velocity you want to shoot.
The better you match the rate to the rond the better accuracy you will get.
get it wrong and accuracy will become very poor at long distances
But there is room in the #'s
for .308 you can shoot 155, 168 and 175 grain all out of the same twist rate, but the accuracy will fall off as the distance increases, about 600 yards for 168gr and 800 yards for 175gr, out a 20" inch barrel, increase the barrel length will increase the velocity keeping the bullet stabilized longer, so 24" barrel may get you to 1000yards with 175gr