In US v. Miller, the Supreme Court found that the NFA was constitutional because short-barreled shotguns have no military uses. Assuming for the sake of argument that's true (it's not), how can the machine gun clauses of the NFA be constitutional? Nobody would say machine guns have no military uses; in fact I'd guess weapons meeting the legal definition of machine guns are more common in the military than other weapons. So what gives?
Because that’s not what Miller says
The court evaluated and expanded Miller in District of Columbia v. Heller, 554 U.S. 570 (2008) at p.52:
Read in isolation, Miller’s phrase “part of ordi nary military equipment” could mean that only those weapons useful in warfare are protected. That would be a startling reading of the opinion, since it would mean that the National Firearms Act’s restrictions on machineguns (not challenged in Miller) might be unconstitutional, machineguns being useful in warfare in 1939. We think that Miller’s “ordinary military equipment” language must be read in tandem with what comes after: “[O]rdinarily when called for [militia] service [able-bodied] men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time.” 307 U. S., at 179. … We therefore read Miller to say only that the Second Amendment does not protect those weapons not typically possessed by law-abiding citizens for lawful purposes, such as short-barreled shotguns.
And at p.54:
Like most rights, the right secured by the Second Amendment is not unlimited. From Blackstone through the 19th-century cases, commentators and courts routinely explained that the right was not a right to keep and carry any weapon whatsoever in any manner whatsoever and for whatever purpose. …Although we do not undertake an exhaustive historical analysis today of the full scope of the Second Amendment, nothing in our opinion should 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.
We also recognize another important limitation on the right to keep and carry arms. Miller said, as we have explained, that the sorts of weapons protected were those “in common use at the time.” …
It may be objected that if weapons that are most useful in military service—M-16 rifles and the like—may be banned, then the Second Amendment right is completely detached from the prefatory clause. But as we have said, the conception of the militia at the time of the Second Amendment’s ratification was the body of all citizens capable of military service, who would bring the sorts of lawful weapons that they possessed at home to militia duty. It may well be true today that a militia, to be as effective as militias in the 18th century, would require sophisticated arms that are highly unusual in society at large. Indeed, it may be true that no amount of small arms could be useful against modern-day bombers and tanks. But the fact that modern developments have limited the degree of fit between the prefatory clause and the protected right cannot change our interpretation of the right.
“Nobody would say machine guns have no military uses” - indubitably. However, nobody would say that they had a military use in the seventeenth century or that they are “typically possessed by law-abiding citizens for lawful purposes” in the twenty-first. Either of these make the prohibition of machine guns constitutional.