16" overall barrel length is what ATF says is bare legal minimums. So 16" OAL is the technical legal answer unless you want to jump through the hoops and SBR it. You could settle on a flash suppressor or muzzle break which is what I am assuming your doing. One you have that picked out measure its OAL and work with your gun smith. You could then conceivably cut the barrel shorter than 16". Lets say your Flash Suppressor Muzzel Break is 2 1/2" long...your smithy could in theory cut the barrel to 13.5" and permanently attach the FS/MB to the barrel by pinning or a small weld. Hope this helps.

BTW there is also a completed rifle OAL too...26" comes to mind, muzzle to butt plate. STAY LEGAL...

Capt Beach