If it were me (hypothetically, of course) I'd import them through Mexico. All you need is one yacht's worth(easily several dozen) that you could bring over from Somalia.
But of course, they do not really need full auto weapons. Those are only more effective than semi-autos in rather specialized circumstances.
The fact that the Islamists did not copycat the DC snipers shows that either they do not have much operational “reach” or that they are so obsessed with topping 911 that they disdain lesser strikes or that Iraq and Iran have been very effective in the “flypaper” strategy in attracting and disposing of jihadists much closer to home.