Well, a quick look at the amazing internet does seem to indicate one could buy a lot (but have to deal with ATF issues).  I didn't see that grenades, self contained artillery shells or guided rockets were legal.

But to the real point.  I do not see that any even very well "uprising" could possibly ever successfully oppose the US Military, and even if some portion defected, they would still be out gunned.  Or conversely, if ever a domestic force were capable of presenting a challenge, our international force would have had to become so degraded that we'd be just as worried about Cuba, Mexico, or any other small nation successfully attacking as well.  A domestic war would be the least of our problems.