<P> Owning both a short barrel and a legal - length rifle could be construed as intent to build an illegal, unregistered SBR . This possibility was contested and won in the U.S. Supreme Court case of United States v. Thompson - Center Arms Company . ATF lost the case, and was unable to prove that possession of a short barrel for the specific pistol configuration of a Thompson Contender is illegal . ATF later released ruling 2011 - 4 to clarify the legal status of owning such conversion kits . </P> <P> Removal of a weapon from classification as an NFA firearm, such as the reclassification of the original Broomhandle Mauser with shoulder stock from "short barrel rifle" (SBR) to a curio or relic handgun, changed its status as a Title II NFA firearm but did not change its status as a Title I Gun Control Act firearm . </P> <P> Muzzle - loading firearms are exempt from the Act (as they are defined as' antique firearms' and are not considered' Firearms' under either the GCA or the NFA). Thus, though common muzzle - loading hunting rifles are available in calibers over 0.50 ", they are not regulated as destructive devices . Muzzle - loading cannons are similarly exempt since the law draws no distinction between the size of the muzzle - loading weapons; thus it is legal for a civilian to build muzzle - loading rifles, pistols, cannon and mortars with no paperwork, however, ammunition for these weapons can still be classified as destructive devices themselves, such as explosive shells . While an' antique firearm' is not considered a' firearm' under the NFA, some states (such as Oregon) have laws that specifically prohibit anyone from owning / obtaining an' antique firearm' that could not otherwise own / obtain an GCA or NFA defined' firearm' (i.e., felons, recipients of dishonorable discharge from military service, the mentally adjudicated, etc .). </P> <P> Individuals or companies seeking to market large - bore firearms may apply to ATF for a "sporting clause exception". If granted, ATF acknowledges that the firearm has a legitimate sporting use and is therefore not a destructive device . Certain large safari rifle calibers, such as . 585 Nyati and . 577 Tyrannosaur, have such exceptions . </P>

When were machine guns outlawed in the us