It doesn't make much sense to me to lengthen the outter barrel and not the inner one. As soon as the round leaves the inner its going to move and the likelyhood of it hitting the muzzle brake increases greatly.

Buy yourself an airsoft barrel, even a used one from a stock gun, say 6.08mm and resize it appropriately. This will result in better accuracy and more velocity.

The "search" feature is your friend. Plenty of information on this right here. Good Luck!