I don't have any firsthand experience in using an NFA weapon in an otherwise lawful self-defense shooting, but you should be aware that you are just giving the DA ample ammunition (no pun intended) to paint you as slightly off-kilter to 12 people who were too stupid to figure out how to get off jury duty.
You're going to have a world of hurt any time you use deadly force against someone, regardless of the circumstances. It will likely cost you a lot of time and money to survive unscathed. Why make the situation worse by having to explain to a courtroom full of idiots that machineguns are legal? Sure, the DA may say, "they're legal, but why would anyone need one?"
Do you really want that going through the minds of the jury when they're back in the jury room? I sure wouldn't.
You could also probably forget about your CLEO ever signing for you again if you whacked somebody with something he or she signed for.
And, do you have any idea how loud a machinegun is going to be inside?
Ok, I know I focused on machineguns here, but I'll wager the same things go with AOW, Cans, etc.
James Bardwell mentioned this a few months ago on another board - I'm not sure if he cited a specific case or if he said it was generally considered a bad idea...