Under the National Firearms Act (NFA) it is illegal for a private citizen to possess a sawed-off modern smokeless powder shotgun, i.e. with a barrel length less than 18 inches (46 cm) and an overall length less than 26 inches (66 cm), without a tax-stamped permit from the Bureau of Alcohol, Tobacco, Firearms and Explosives, which requires a background check and a $200 fee for every transfer. However, if the weapon was manufactured by a licensed builder, with a short barrel and no stock, the transfer fee is $5. [1] (Short-barreled muzzleloading blackpowder shotguns, in contrast, are not illegal by federal law and require no tax-stamped permit, although they may be illegal under state law.) As with all NFA regulated firearms, a new tax stamp must be purchased before every transfer. Inter-state transfers must be facilitated through a Class III Federal Firearms Licensed (FFL) dealer while intrastate transfers may be between two persons.[6]
http://www.law.cornell.edu/uscode/html/uscode26/usc_sec_26_00005845----000-.html(a) Firearm
The term “firearm” means
(1) a shotgun having a barrel or barrels of less than 18 inches in length;
(2) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length;
(3) a rifle having a barrel or barrels of less than 16 inches in length;
(4) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length;
(5) any other weapon, as defined in subsection (e);
(6) a machinegun;
(7) any silencer (as defined in section 921 of title 18, United States Code); and
(8) a destructive device. The term “firearm” shall not include an antique firearm or any device (other than a machinegun or destructive device) which, although designed as a weapon, the Secretary finds by reason of the date of its manufacture, value, design, and other characteristics is primarily a collector’s item and is not likely to be used as a weapon.
