An assault rifle, as in a military style assault rifle (think AK-47 or M-16) is essentially the exact same as a machine gun. Fully automatic, not semi-automatic. For all intents and purposes, you could interchange "assault rifle" and "machine gun" and still be correct. As was mentioned earlier, there are ways to modify a semi-automatic rifle, like an AR-15, to make it fully-automatic. Which is illegal, but that doesn't really stop people from doing it. If we are talking about legislation, one law that I think makes sense is to outlaw firearms that can be modified to be fully automatic, even if it is original sold as a semi-automatic. Semi-auto hunting rifles and shotguns are not the kinds of guns that you can modify to shoot as fully-automatic.
I didn't think that fully automatic rifles were legal, even in Nevada, I thought it was a federal law that outlawed them.