Back in the day someone was selling modified pump shotgun bbls in Shotgun News. They machined away the top of the bbl at about the 14" mark forward, leaving a long tapering section of the bottom intact out to the 18" mark. Essentially a 14" "sawed off shotgun" with a permanently affixed spike bayonet, the theory being the "end of the bbl" was still legal length. ATF disagreed, they said the "end of the bbl" was at the short end of that spike, at the 14" mark. ATF measures from the shortest possible point at the muzzle, so say an AK slant brake was permanently attached to make legal length, better make sure the measurement is from the short side!
That is one of the reasons you always see 16 and 1/2" bbls and 18 and 1/2" bbls from the factory, no one want to get a ding in a muzzle and have the ding be below the legal limit. It also leaves a bit of leeway for recrowning and such.