Barry, depends all firearms are different. I've had 1:9 twist ars like heavier 69-75 grain ammo and 1:7 twist like 55 grain ammo. For general rule of thumb normally 1:9 twist will do well with lower grain ammo like 50-62. Then 1:8 will do best with 62-69 grain but is the most versatile twist. Then 1:7 twist like heavier 69+ grain ammo. Again doesn't hurt to try as many grains and brands of ammo you can find. Some heavier ammo your 1:9 twist barrel won't like and some it will. Guessing you're talking about a AR-15 if so great thing about them is it's very simple to switch barrels. So down the road you can swap barrels to a 1:7 twist to help stabilize heavier ammo. Hope this helps.