However, machine guns manufactured domestically prior to 1986 or imported prior to 1968 may be transferred between civilians in accordance with federal and state law.
You seem to miss that part. There are plenty of automatic weapons circulating to go around. The ban on assualt weapons expired in 2004 anyway. Of great concern is that there are immeasurable amounts of handguns already in circulation and (literally) tons more being produced at alarming rates every day. It's a fucked situation that our government refuses to address.
http://www.nationalreview.com/kopel/...0409130630.asp
the now-expired Assault Weapons Ban of 1994–2004...
Before the ban elapsed in 2004, automatic rifles sold at guns shows were primarily purchased without the firing pins. These are obtainable on the black market, usually from the dealer on the sly. Hell, I think you can easily just order the firing pins for whatever make and model from Soldier of Fortune magazine.