No firm rule on this. Try all you can. I get every good accuracy in a 1 in 9" CZ barrel using 65 Sierra GK and 69 MK. The 68 Hornady Match also does well. Conversely, a couple of Colt ARs with 1 in 7" barrels shoot 55 grain bullets as accurately as they do heavier ones. I'm talking about good quality bullets here, not cheap bulk stuff that usually doesn't shoot real well in anything.