okay, let's assume full auto smgs were as easy and cheap to buy as their neutered cousins. i'd say sure, provided you have decent training. a semi version is less than ideal, though, i think. you have the same size and weight of a rifle carbine with an underpowered cartridge.
i'm not sure why gun owners automatically assume that you're going to start foaming at the mouth and wildly spraying just because you have a fun switch on your weapon. this is like the gun nazis that think a normally safe, responsible adult will turn into a homocidal maniac if given a gun. it also reminds me of debates between bolt action owners and owners of self loaders. if you have fire discipline with a bolt, you can apply fire discipline with a semi. if you have fire discipline with a semi you can with a full auto.
short controlled bursts can be very effective at close range. at home defense distances, every round of a short, controlled burst will impact the target in the thoracic cavity if you apply the same training as you would with any other firearm.
i think it was mas ayoob who popularized the idea that politically incorrect firearms will get you into more trouble. i disagree. this may be true in some less enlightened districts but for the most part, the questions at hand will be whether you were justified in shooting, whether you stopped shooting when the threat was neutralized (jurys tend to frown on "anchoring" shots) and whether you hit anyone else's property or persons. if it was an otherwise good shoot, most jurisdictions won't care what tool you use, whether a flintlock or a p90. the civil case is another matter but the questions are the same, it's only a difference of burden of proof and other procedural issues. you will have to hire your own attorney for a civil case but i doubt it will be much more likely to have suit filed against you just because of the tool you use. i also think this idea of the deceased's relatives filing suit is over inflated. people who break into homes are usually poor and uneducated, their families usually follow this pattern. it is exceedingly rare that they can afford an attorney. if they do or find one that will do the work pro bono, advise them and the attorney that you will file counter suits against both. the problem will likely go away.
in short, a true subgun would be an outstanding home defense weapon if it were practical; a pistol caliber carbine isn't well suited to the role because of power limitations but is better than a pistol.
