The ostensible impetus for the National Firearms Act of 1934 was the self-imposed gangland crime of the Prohibition era, such as the St. Valentine's Day Massacre of 1929, and the attempted assassination of President-elect Franklin D. Roosevelt in 1933, brought about by alcohol being illegal.

[9] NFA categories have been modified by laws passed by Congress, rulings by the Department of the Treasury, and regulations promulgated by the enforcement agency assigned, known as the Bureau of Alcohol, Tobacco, Firearms and Explosives or ATF.

AOWs can be pistols and revolvers with smooth bore barrels (e.g., H&R Handy-Gun, Serbu Super-Shorty) designed or redesigned to fire a fixed shotgun shell.

For example, in Ruling 81-4, ATF declared that any AR-15 Drop-in Auto-Sear (DIAS) made after November 1, 1981 is itself a machine gun, and is therefore subject to regulation.

[17] Most current fully automatic trigger groups will not fit their semi-automatic firearm look-alike counterparts—the semi-automatic version is specifically constructed to reject the fully automatic trigger group by adding metal in critical places.

This addition is required by ATF to prevent easy conversion of Title I firearms into machine guns.

For the civilian possession, all machine guns must have been manufactured and registered with ATF prior to May 19, 1986, to be transferable between citizens.

[19] Falsification and/or misuse of the "demo-letter" process can and has resulted in long jail sentences and felony convictions for violators.

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.

[23] Individuals or companies seeking to market large-bore firearms may apply to ATF for a "sporting clause exception".

There are generally three ways to own an NFA weapon: as an individual, through a gun trust, or as a Limited Liability Company (LLC).

An individual owner does not need to be an NFA dealer to buy Title II weapons.

Private owners wishing to purchase an NFA item must obtain approval from the ATF, pass an extensive background check to include submitting a photograph and fingerprints, fully register the firearm, receive ATF written permission before moving the firearm across state lines, and pay a tax.

The payment of a $200 "making tax" prior to manufacture of the weapon, although a subsequent transfer of AOWs after they are legally "made" is only $5.

[33] Upon the request of any ATF agent or investigator, or the Attorney General, the registered owner must provide proof of registration of the firearm.

For example, lawful heirs must submit a Form 5 and wait for approval before taking possession of any NFA item willed to them.

§ 2201a) to allow Nuclear Regulatory Commission (NRC) licensees and authorized contractors to possess machine guns for the purpose of providing security.

[39][40] Importation of NFA firearms was banned by the 1968 Gun Control Act which implemented a "sporting" clause.

The domestic manufacture of new machine guns that civilians could purchase was effectively banned by language in the Firearm Owners Protection Act of 1986 (also known as "McClure-Volkmer").

Machine guns made after 1986 for law enforcement but not transferable to civilian registration are usually priced only a few hundred dollars more than their semi-automatic counterparts, whereas a pre-Hughes Amendment registered machine gun that can be legally transferred commands a huge premium.

Some types of NFA firearms can be relatively simple to make: a Short-Barreled Rifle can be made by swapping out the upper receiver for one containing a short barrel, and a Short-Barreled Shotgun can be created by using a pipe cutter to shorten the barrel length.

Under regulations, use of a trust allows prospective purchasers of NFA items to avoid some of the federal transfer requirements that would otherwise be imposed on an individual.

In 2013, ATF proposed new rules,[45] often referred to as ATF Proposed Rule 41p, which, if adopted, would require all "responsible persons" of an entity being used to purchase NFA items to comply with the same procedures as individuals in obtaining NFA items.

In an NFA trust, a responsible person is defined as "any grantor, trustee, beneficiary, ... who possesses, directly or indirectly, the power or authority under any trust instrument, ... to receive, possess, ship, transport, deliver, transfer, or otherwise dispose of a firearm for, or on behalf of, the entity.

The previous requirement for "chief law enforcement officer" approval was eliminated, while all responsible people for a trust must now comply with the same restrictions as individual owners.

The defendant Miller had been arrested for possession of an unregistered short double-barreled shotgun, and for "unlawfully ... transporting [it] in interstate commerce from Claremore, Oklahoma to Siloam Springs, Arkansas" which perfected the crime.

No evidence that such a firearm was "ordinary military equipment" had been presented at the trial court (apparently because the case had been thrown out—at the defendants' request—before evidence could be presented), although two Supreme Court justices at the time had been United States Army officers during World War I and may have had personal knowledge of the use of such weapons in combat.

Subsequent rulings have been allowed to stand, indicating that short-barreled shotguns are generally recognized as ordinary military equipment if briefs are filed (e.g., see: Cases v. United States),[50] describing use of short-barreled shotguns in specialized military units.