ATF method of measuring shotgun or rifle barrel: Close the breech on an empty chamber. Drop a metal rod down the muzzle until it is resting on the closed breech.. Tape the rod at the muzzle. Take the rod out of the barrel. Measure the distance on the rod, which will be muzzle to breech face. Barrel length includes the part of the barrel that fits inside the receiver. My M6 Scout .22/.410 has a factory barrel within a hair of 18" even tho published sources list 18.5". ATF allows factory .22 AR-7 rifle barrels to be 16 inches minus the thickness of a .22 cartridge rim. I suspect that is a special dispensation on ATF's part for factory M6 and AR-7. Locally, a guy bought a Winchester Model 12 from a prison surplus auction. He discovered the barrel was shorter than 18 inches. He contacted the local office of federal ATF for advice in what to do. They advised him to take the barrel off the gun and take the barrel to a gunsmith to have a permanent extension installed. I believe he had a Cutts Compensator (recoil reducer, choke) permanently installed. You will find permanently attached compensator tubes, choke extensions or just iron pipe welded on too-short shotgun barrels to bring them to legal length. I read a query once about why would someone weld a section of 10ga shotgun barrel over the end of a 20ga shotgun barrel.