Is it legal

[joeyj]

A friend has a single barrel 12gauge and he took it to a gunsmith (Who has since passed away) to have the barrel shortened. It is now 18 1/2 inches long.Saw a fellow on YouTube who said you needed to have a paper from ATF which gives you permission to shorten it. My question is "Is what he has legal or not".. Thank you

[bogey1]

Yes it's legal.

[USMC3531]

As long as the barrel is 18"+ and the gun is 26" + over all its legal

[Xringshooter]

The only time you would need to get "permission" to cut the barrel down is if you were making a SBS (short barreled shotgun - which is a shotgun with a barrel shorter than 18 inches or an overall length of less than 26"). Then to get the "permission" you have to fill out and file a Form 1 (Application to Make and Register a Firearm) and pay the $200 tax stamp. But you can't cut the barrel and make a SBS until AFTER you receive the approved Form 1.

[bortle]

As long as the barrel is 18"+ and the gun is 26" + over all its legal

This is correct for a shotgun. Measured from the breach face to end of barrel.
Be advised it will have no choke and be fun as hell to shoot