During the “Gangster Era” of the 1920s, the Feds passed the National and Federal Firearms Acts of 1933 and 1934, and it is clear their aim was to attack the ownership of any concealable weapon that related to guns like the tommy gun, with its removable stock, or the various factory-made shotgun pistols, like the all-steel double barrel made in San Francisco, or the H&R Gamegetter, as well as folding stock shotguns like the Marble’s with barrels less than 18" (The marble variant with 18" barrel is legal). They didn’t want to get into the uproar and court cases that would have occured had they just tried to ban convential rifles, shotguns and pistols, for sure. So, they set minimum barrel lengths for a shotgun or rifle (18 inches, since reduced for rifles to 16 Inches) and overall length (26 or 26-1/2", I forget which). They also used manufacturing licensing laws to get at some guns. For example, it was considered that if you put a shoulder stock on a pistol, your were “manufacturing” an illegal short-barrel rifle from a pistol, with no manufacturer’s license. The same was true if you cut a rifle down into a pistol. Shotgun pistols were flat-out illegal, but if say winchester had made a pistol version of their bolt action .22 rifle, and sold it initially as a pistol, it would have been legal, whereas the same action on a rifle, cut down by an individual to pistol size, would be illegal.
There was nothing in the laws to make illegal the use of shot cartridge forms of ammunition in conventional metallic calibers fired through arms with rifled barrels. The minute you smooth-bored a gun (also a pain for criminalists once the science of idnetifying specific guns by rifling marks on the bullets came in), it becamse a shotgun, and in the cases of handguns, an illegal shotgun pistol.
Very complicated and very unnecessary. Despite some very dramatic crimes like the St. Valenitne’s day massacre, committed with tommy guns and sawed-off shotguns (maybe even legal shotguns, come to think of it; I don’t really know and shouldn’t have made that statement), nationwide, murder rates were not extraordinary and in the case of the gangsters, it was usually one criminal killing another. The St. Valentine’s Day Massacre caught up a couple of semi-innocent guys in it, and caused enough public outcry to start the Feds looking for laws dealing with weapons they didn’t like in private hands.
There were transfer provisions built into the law, as well as amnesty ownership for those that possessed the illegal-type guns before the law was passed. At the time, it was very hard in the USA to get a law passed totally outlawing anything to include items owned before the law was passed. That is called an ex-post facto law and is illegal. Despite that, today, officials don’t seem to care if the violate the Constitution with laws, and many judges don’t, and the result is, for example, California has had some success in outlawing items and seizing those owned before the law was passed, or making one remove them from the state. In some instances, they were amnesty-grandfathered in, but in a couple, like threaded muzzles on handgun barrels, or assault rifles that missed the first writing of the laws by accident and were added later, perhaps after someone had gone out an bought one since it was not on the list, they were NOT grandfathered in! That is clearly an illegal ex-post facto law, but the courts would do nothing about it.
Very complicated. One must almost be an expert in firearms law these days just to safely own a gun or ammunition in some states.
Today, legally registered machineguns and the like, owned by private citizens in the states that allow them, have been involved in almost zero criminal activity of any kind.