I do believe that the standard for shot guns Is 18" or longer and 16" for rifles, Or very close to that!
Did not the ATF set up Randy Weaver back In 1992, In or on Ruby Ridge , Idaho, thay had him cut the barrol of a shot gun down to 17-1/2" or something like that. Just another case of our great Goverment looking out for us:barf: Semper FI to all, Hank D.