Once upon a time in America, owning a machine gun was as easy as this: promising to "be responsible for their resale only to those on the side of law and order," according to a trade notice posted by the Auto-Ordinance Company, manufacturer of the weapons used in the St. Valentine's Day massacre. An alleged gun runner sat down with a Tribune to explain the one-step process:

"It is no trouble to buy machine guns," Daniels told a reporter. "All I had to do was to send to New York for them and they were shipped to me. I was getting them for the Mexican rebel army."

V.A. Daniels, alias Danielski, told a Tribune reporter this after having been arrested for flipping 15 Thompson submachine guns at a profit of $269 to $394 each ($3,600 to $5,300 in today's dollars) to beer runner "Red" Bolton, mail robber Charles "Limpy" Cleaver, and beer-gang chief Joe Saltis. It was May 1929, a couple months after the massacre that defined the gangster era in Chicago, and the deputy police commissioner claimed that Chicago gangs owned 500 machine guns.

State Senator Adelbert H. Roberts, in the wake of the massacre, moved to ban machine guns for all but cops, guards, messengers, and members of the armed forces; John Stege, the deputy commander, wanted them banned completely. It would happen, eventually, but there was resistence, and like all gun control, it would take years.

Leading the charge was the Tribune editorial page, which wrote, in 1934:

In the revolutionary war the people were able to gain their liberties because when they tried for them possession of firearms was common and many of the citizens knew how to use them. A disarmed population of people familiar with weapons would not have had much chance. In 1789 the weapons in general use would be long rifles, muskets, and clumsy pistols. The people were entitled to have the best weapons they could make or purchase. Now the best weapons for individuals are machine guns and automatic rifles. Use which can be made of these is indicated by law, but it is not the possession of which is properly an offense under the constitution.

[snip]

It is notorious that when restrictions are put upon the possession of firearms or any particular kind of weapon they never are effective against the criminal classes but only put the peaceable man at a disadvantage or in a false position before the law. The prohibition does not bother the enemy of society but it makes a technical offender of the decent citizen. The man who would not misuse a weapon is the man who is injured. The drive for public security is thus given the wrong direction.

In other words, when Tommy guns are outlawed, only outlaws will have Tommy guns, and the peaceable citizen will not have the ability to return machine-gun fire when needed.

By 1934, Illinois had passed a law banning the private use of machine guns, and the National Firearms Act of 1934 did the same. Only the NFA wasn't a strict ban; it was a tax, slipped through like the ACA as a revenue act and adding a $200 tax (about $3,500 today) on "banned" weapons, effectively doubling the price of a machine gun and adding expensive annual license taxes as well.

And it got its test as constitutional law thanks to Jack Miller, a member of the O'Malley Gang, famed for pulling off one of the few simultaneous heists of two banks. After the gang was finally broken by law enforcement in 1936, Miller was picked up with an unregistered short-barreled shotgun, one of the weapons so heavily taxed by the NFA. So the U.S. attorney brought him up on charges of what amounted to tax evasion. The case made it to the Supreme Court as U.S. v. Miller, in which it was decided:

In the absence of any evidence tending to show that possession or use of a 'shotgun having a barrel of less than eighteen inches in length' at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. Certainly it is not within judicial notice that this weapon is any part of the ordinary military equipment or that its use could contribute to the common defense.

And that's basically all it said: the military doesn't use sawed-off shotguns, so you aren't allowed to have them, unless you pay the tax. In some ways, it's significant, because it upheld the right of the government to regulate certain classes of weapons, as Scalia wrote in District of Columbia v. Heller:

The Court’s opinion should not be taken to cast doubt on longstanding prohibitions on the possession of firearms by felons and the mentally ill, or laws forbidding the carrying of firearms in sensitive places such as schools and government buildings, or laws imposing conditions and qualifications on the commercial sale of arms. Miller’s holding that the sorts of weapons protected are those “in common use at the time” finds support in the historical tradition of prohibiting the carrying of dangerous and unusual weapons.

If law-abiding citizens had seen fit to stock up on Tommy guns back in the day, they might be legal now. But since they weren't in "common use," and therefore the sort of weapon you could expect an average Joe to have in his closet when the militia was called up, prohibitions against them are constitutional—even though sawed-off shotguns, the basis of Miller, were a common weapon for Confederate soldiers to equip themselves with.

The principle of Miller is basically "if only outlaws use this gun, we should probably outlaw it." Which means it's a bit of a mess, as Brian L. Frye writes in "The Peculiar Story of United States v. Miller" (PDF):

So what did Miller hold? At a minimum, it held the Second Amendment permits Congress to tax firearms used by criminals. At the maximum, dicta suggest the Second Amendment protects an individual right to possess and use a weapon suitable for militia service. And in general, it implies the Second Amendment permits reasonable regulation of firearms. In any event, the Court left legislators a lot of wiggle room.

But what does Miller tell us about the meaning of the Second Amendment? Maybe nothing…. Nevertheless, faint-hearted originalists and incrementalists alike might find Miller useful, or even appealing. After all, it anticipates the status quo: federal, state, and local governments may reasonably regulate firearms, but may not prohibit them altogether.

Miller basically punts, but in a way that makes most everyone happy, appealing to the masses by default: if a lot of people use a particular gun, it's probably okay.