The main reason that they ruled the way they did was because Miller's whole defense was along the lines of, "Because, uh, yeah... the Constitution and stuff...please don't convict me?"
He didn't present any evidence, so they ruled against him. He could have argued the usefulness of shortbarreled shotguns as used in WWI, and how short barreled weapons in general proved useful in tight areas, and why a militia might need such weapons to clear houses and such. However, he didn't.
If you got convicted of a similar offense, you could make a much better argument than he did, but would you really be willing to risk it turning sour?
>>>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. Aymette v. State of Tennessee, 2 Humph., Tenn., 154, 158.<<<
Here's the Miller Case:
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=307&invol=174