Barrel twist is the inches the bullet travels to do one full rotation. 1:8 twist means the bullet traveled 8″ to do one full turn. We like 1:8, as its the closest to what the average shooter will need. If you look at the picture above, this is the best chart that I can find to help show you what twist rate is best for you. The “gr” or “grain” of the bullet is the weight. The heavier the bullet; less spin is better. The lighter; the more..thats it. This does not mean you cannot shoot a 45 grain bullet through a 1:7 twist. This means it will be more accurate** long distance if you use something with more twist rate.
If you are like most of us and bought a mountain of bulk .223 or 5.56 ammo during one of the many “scares”. Then you may want to take a look at what grain you have. This will help you selecting the twist rate you want. I have personally shot 45 gr. through a 1:7 then put it through a 1:9 and guess what, at 100 yards, there was no difference. If you are shooting long distances, then you will notice a some difference. Don’t beat yourself up over this when deciding what AR-15 upper to purchase.
**If you buy cheap barrels, the twist rate will not matter as you will not be able to hit anything 100 yards accurately. This is because there is a lot that goes into making barrels accurate. Twist rate is just one of many variables that go into making good AR15 barrels