I know several people, mostly professionals, who use NFA weapons as part of their defensive (or offensive in certain cases) menu. Weapons include several Short barrelled shotguns, a MAC-11 and a couple Short barreled rifles.
Myslef, I will go back to carrying my Beretta 25 in a wallet holster once my paperwork comes back, afterwhich it will be a properly registered AOL, thus an NFA weapon.
If you have copies of your paperwork and are in your home state (and obviously the weapon is legally registered, etc..) you shouldn't have nay additional concern from using an NFA weapon in self defense.
Obviosuly, if you empty a full mag from a MAC-10 in the middle of the mall because someone pushes you, you've got a problem. But if you pull out your Glock 20 and fire sixteen rounds at a guy that cuts you off on the highway you've got a problem too. (BTW- we had a road rage murder the other night....be careful during the holidays!)