I am not a big full auto/select fire enthusiast. Even in actual military use, it serves a narrow purpose, i.e. area denial and the like, that can be often be met with semi-automatic fire.
But, if a state like, say, Montana or Texas, under the authority of the 2A, were to officially activate their state militia, they could authorize citizens to have nearly anything the state wished to approve.
There is much doubt with regard to individuals comprising an unorganized militia being so armed (and such legal challenges have utterly failed), but the right of a state to so arm its militia to fight against tyranny is beyond dispute.
This is the only legitimate circumvention of the NFA of which I am aware. It remains an option of individual states should the need arise.