Good info Mooseman.
To clarify for the OP, an SOT is a dealer that has paid a special tax to BATFE to be able to sell and buy silencers, machineguns, short barrel shotguns/rifles, destructive devices and All Other Weapons (AOWs).
Since civilian (non-SOT) individuals are not legally able to buy machineguns manufactured after May '86, the SOT is able to buy these guns for demonstartions to local police departments, hence the demo request letter from the CLEO (chief law enforcement oficer).
This is a very simplified explanation and there are many variences in the law. You should consult a Class 3 FFL holder.
And for Cyrano, you absolutely CAN buy a full auto AK47 legally as a civilian. I own a full auto M119mm with a Bowers CAC 9mm silencer and a Walther P-22 with a YHM silencer.
Check out the info at
MACHINE GUN DEALER, MACHINE GUNS FOR SALE . They have a FAQ that will tell you what is required to buy these weapons (it aint that hard, just expensive and time consuming).