A lot of people at ranges across America are probably couldn't tell you what the barrel twist rate is or their rifle without looking. As noted above, barrel twist is the number of inches spanned in one complete rifling rotation- iow how fast is your projectile spinning by the time it exits the muzzle? Twist rate is commonly noted as a ratio- for example 1:8 (the preferred 300 BLK twist rate- especially if you want to shoot subsonic ammo). Barrel Twist also has an effect on the projectile weight you use:
Common barrel twists for 5.56 AR-15 rifles:
1:9 – does not stabilize heavier projectiles well (55gr and up- you will see 'keyhole' hits on your targets); stabilizes lighter projectiles extremely well (like varmint bullets)
1:7 – stabilizes heavier projectiles well ; stabilizes light projectiles well enough. You can't go wrong with a 1:7- that's what the US Military uses.
Common barrel twists for .308 Bolt Action rifles
When shopping for bolt actions twist becomes an important consideration if you want to be able to use and experiment with subsonic loads. Bolt actions can have different twist rates (and barrel lengths – which also factors into stability with heavy projectiles) than the ones listed here so it's worth some investigation before you make a decision.
1:16 – will not stabilize a heavy subsonic projectile (200gr and higher) in .308
1:10- will nicely stabilize a heavy subsonic projectile and also stabilize lighter projectiles well enough
Common barrel twists for 300 BLK (AR-15 platform) rifles
1:8 – this does the trick and supports whatever you send downrange regardless of the muzzle velocity
1:10 might stabilize a 200gr projectile at subsonic speeds but you are severely limited in projectiles with a 1:10 BLK barrel
The bottom line here is that you should be certain that your 300 Blackout kit has a 1:8 barrel- there isn't a good reason not to.