What about 18.5" barrels with a sort of "Vang Comp" (to borrow from the shotgun world) for rifles. Port the end of the barrel like a flash hider or compensator. Mill the outside near the muzzle so it looks like a flash hider or compensator. It would be a lot of precision drilling mind you. If you did that process BEFORE cutting the rifling, it wouldn't mess up the rifle grooves near the muzzle.
Remington did something like that with their Toblerone shaped barrels on the 700 VTR.