Even if it's legal, well and good, once it hits the public venues that someone shot someone (no matter the circumstance) with a machine gun, the anti's will have a field day with it.
Maybe, maybe not. Depends on the situation. If you are fending off multiple well armed attackers, it may very well be appropriate force. Read the Bekwith Incident. http://www.afn.org/~guns/ayoob.html
In any event, the question was "is it legal". And the answer is: "yes it is".