So I am about to BUILD my first AR. I've owned several but never actual built one. I want to install a Diablo flash hider but don't want it too far from the handguard. I prefer to have it flush with the handguard to hide the barrel itself. Now my question is this: With a 12 inch handguard (with a low profile gas block underneath) what size barrel would I need to obscure the barrel taking into consideration the threads on the barrel. A 12.5"?