Re: M40A1 barrel
If you are going with a M40A1 clone use a Hart or H&S Precision 1:12 twist.
Sure, you can use different barrels, scopes, bases and stocks, but then it's not an M40A1.
IMHO you are going to see little to no accuracy difference with any other top end barrel maker. It's going to matter more who installs, chambers and trues the action.