Minimum barrel length is 18" with appropriate stock length (there is a limit on overall length as well - not sure but I think it is 32").
That's why when you see pistol gripped shotguns at gun shows, they typically have a longer barrel than the shorty 18" home defense versions.
Depending on model (Mossberg, Remington, etc.) you can buy commercial 18.5" barrels at Cabelas or most any gun store. Just watch for overall length requirement when replacing barrels.
"Limit politicians to two terms. One in office and one in jail!" (Borrowed from an anonymous donor)
It sounds like you may have a Remington 870 "Special Purpose". The 21" barrel length is 3" over the legal minimum and many people prefer it for easier handling in tight places. With modern loads, very little muzzle velocity loss out of the 21"er. Screw-in choke tubes make 'em quite versatile.
"Short-barrel firearm" means a rifle with a barrel length of less than 16 inches or a shotgun with a barrel length of less than 18 inches, or any weapon made from a shotgun or rifle if, as altered, it has an overall length of less than 26 inches.
Jason
NRA Life Member
TSRA Life Member
"No man stands so tall as when he stoops to help a child."