I'd argue if you have a second amendment right to own a miltary style assault weapon capable of firing two hundred rounds per minute and designed for no other purpose than killing large numbers of people in a short amount of time, you also have a second amendment right to own a fully automatic weapon which is designed for the same purpose. But let's not stop there, in a broader interpretation of your constitutional right to bear arms, a claymore mine is arms, so is a surface to air shoulder fired heat seeking anti-aircraft missile. You should be able to buy as many fragmentation grenades as your budget will allow and let's not overlook anti-personnel pressure and proximity mines, those are clearly arms too.
Alas however, the reason you can't is because as Scalia wrote in the Heller opinion, the gov't has a Constitutional Right to determine which "classes" of weapons a citizen may possess.
Meantime, why is it in Texas I have a constitutional right to openly carry an AR-15 with a high capacity magazine into a grocery store but a patriot can't carry one into a Governor's news conference or into the Texas House of Representatives, gee whiz wally, it's just a hunting rifle and 20,000,000 of them never killed a Governor or Texas or Speaker of the House.
Why heck, hundreds of people probably died today and not one was in a passenger plane shot down by a shoulder fired AA missile, or in a grocery store when someone lobbed a frag grenade through the door. Not one kid stepped on an anti-personnel mine on her way to school this morning, so those should be legal to buy and possess too.
If the gov't can prevent my owning a missile launcher isn't that a slippery slope to them taking my pistol?