Final answer:
A short-barreled shotgun is defined as having barrel(s) under 18 inches or as any modified shotgun under 26 inches overall. The National Firearms Act of 1934 and subsequent laws regulate these firearms. The Supreme Court's ruling in United States v. Miller reinforced limits on their legality.
Step-by-step explanation:
A short-barreled shotgun is defined by having one or more barrels less than 18 inches in length or any weapon made from a shotgun with an overall length of less than 26 inches.
This falls under federal law, particularly the National Firearms Act of 1934. The act was one of the first federal gun control laws and initially arose in response to organized crime incidents commonly associated with the prohibition era. In the landmark case United States v. Miller, the legality of these weapons was challenged, and the Supreme Court ruled that sawed-off shotguns were not protected by the Second Amendment because such weapons were not relevant to the maintenance of a "well regulated militia."
Further laws such as the Brady Handgun Violence Prevention Act, which introduced background checks, came into force following an assassination attempt on President Ronald Reagan in 1981. These and subsequent laws are part of a history of evolving regulations addressing gun ownership, usage, and safety.