Are you confusing the post-2013 MD HBAR rules with NFA? No requirement for HBAR on pistols or SBRs. HBAR is required for MD AR15 Title 1 rifles made today (in addition to 16” OAL barrel).
I was thinking of a 14.5 barrel pinned and welded to over 16 inches with one of the muzzle devices that are slim enough for you to slide the gas block over them so I would have to consider barrel profile. Not sure about OP.