I'm building an Aero M5 (standard, not -A1) and using Aero's Atlas handguard and barrel nut. The barrel nut bottoms out on the receiver when I thread it in... or, actually, it hits both the barrel flange AND the receiver at, basically, the same timing. When I go to torque this thing down, it will be against both the barrel AND the receiver. Is that correct? Edit: it is a Ballistic Advantage barrel.