The argument that you must be able to use something simply doesn't stack up against the Second Amendment. If I want to own an elephant gun just because it is beautiful and looks good over the mantle or because it has sentimental value, it does not follow that I should expect to be able to legally shoot an elephant in my lifetime. In fact, none of the guns in our house have ever been used to shoot anything other than targets in a structured firing range.
Banning a weapon in one state, however, makes little sense if anyone is planning to use a weapon for illegal purposes. Criminals do not usually purhase their weapons legally. Washington DC has the toughest gun laws in the country and one of the highest gun crime rates.
OE is right, though, that there are classes of weapons that must remain in control of the military and would not be appropriate for the average U.S. citizen. An intercontinental ballistic missile would certainly head that list.
I'm not vouching for the accuracy of this site:
http://www.gvps.org/weapon.htm
but it looks like the statutory list of illegal weapons in Texas, or weapons that are illegal under certain circumstances. I believe it is pretty consistent with the laws regulating firearms and weapons in most states.
So it's all in a matter of degrees. Does a 50mm gun constitute a sufficient risk to the public that it should be issued only with special permits or not allowed at all? It does seem the capability of this weapon does put it on a par with machine guns or live grenades with grenade launchers that most of us are not allowed to own.
Weaponry has advanced to the point that the citizens of the free world are again vulnerable to whomever controls the military. Neverthless, should the U.S. ever be invaded by a foreign power, I think all those deer rifles and shotguns would become a significant factor.