If you neighborhood is soooooo bad that you feel a high probability that you will be attacked by squads of bad guys practicing CQB then...... YOU SHOULD MOVE!
That being said I think it is a bad ideas on multiple fronts.
1)I have seen what automatic 7.62 fire does to a human body. It isn't pretty. Imagine housewives and hippies on the jury seeing crime scene and autopsy photos. God help you the BG catches a burst to the face (eye/nose trauma, rounds shattering teeth/jaw, tissue destruction, etc). Does any of this affect the facts of the case? No. Will it affect the jury emotionally? You are an idiot if you think otherwise.
2)In a fight/flight scenario everything slows down. While it may see like you had your finger on trigger for <1sec likely it is 2,3,5 seconds. Firing 10, 20, 30+ rounds into the target area (even if they don't all hit) isn't going to seem "reasonable force" to a jury some of which are neither gun owners nor have they even shot a weapon (much less an automatic).
3)You likely have other weapons in the house (pistol, shotgun, etc). DA will put one some expert witnesses like a police investigator saying that "in my expert opinion I believe the home invasion could have been stopped with less destruct weapon like one of the [INSERT # HERE] the defendant had access to". Maybe they also bring in a military expert stating the purpose of an automatic weapon is suppressive fire. The goal of automatic weapon fire is to suppress the enemy so that other marksmen can engage in accurate fire with less danger to themselves. "Is there are legitimate reason to need to suppress a single target with 30 rounds, sir?". No in my opinion if the defendant had the ability to engage the target with the automatic weapon he could have in the same situation stopped the threat with accurate rifle/shotgun/pistol fire.
4)You may win. To do so you may need to take 2nd mortgage on your house and raid your IRA/401K. It is worth it though. Right? When you are a 67 year old greeter at Walmart you will be glad you wasted $20K, $50K, $100K or more defending both the criminal and civil suits. If you lose the civil suit you will be happy to pay BG family $100K, $200K, $1mil over the next 20 years. At least you are not in jail right?
Basically I see two scenarios
1) You actually need automatic fire to defend yourself. Nothing else will work. Ignoring SHTF, End of World, or Zombie scenarios, if this happens to you a lot you need to re-evaluate your neighborhood, actions, friends, and lifestyle.
2) You didn't need to use automatic weapon and did so "because I can". You may win and still lose.