http://www.findarticles.com/p/articles/mi_m0BTT/is_168_28/ai_112685749 Old, but a good read. This guy got SCREWED by an overzealous prosecutor because he used a legal Class III weapon (Ruger AC-556) to defend himself where he was clearly justified. Sorry if this is a repost.
However, in the state of GA I believe that if one was to use a NFA Weapon in self-defense and the self-defense was justified, then that person would be afforded the same exemptions and protections in O.C.G.A. 16-3-21 as a person who used a non NFA Weapon. O.C.G.A. 16-11-162. Exemption for use of force in defense of others. This part shall not apply to persons who use force in defense of others as provided by Code Section 16-3-21. This part is intended to supplement not to supplant Code Section 16-11-106. "This part" being referred to as this code section; O.C.G.A. 16-11-160. Use of machine guns, sawed-off rifles, sawed-off shotguns, or firearms with silencers during commission of certain offenses; enhanced criminal penalties.
No, there is nothing per se illegal about using a NFA weapon in self defense. The issue in the article was that fact that a politically motivated DA tried to capitalize on public perceptions about fully automatic weapons. Even my ultra liberal criminal law prof was shocked that any DA would pursue a case against this guy since the situation here is pretty much text book self defense.