Forgot to mention, back in the late 90's I was interested in building a Class 3 collection. Over a few years I bought five Form 4 machine guns through a Class 3 SOT that ended up being a good friend. His collection was prodigious and I was allowed to shoot anything I wanted EXCEPT his AR-18. He was worried about bolt failure because it had cracked one in the past.
ETA: Jerry (my C3 SOT friend) had Colt Thompsons, Savage Thompsons, Owen, Austen, MP38, MP40, MP41, Beretta 38, 1919, M60, five or six M16 variants, MP5, MP5k, MP5SD, SW76, Suomi, Valmet, Sten Mark IV and V, M3 Grease Gun, M2 Carbine, BAR, MG42, Sturmgewher 43, Madsen M50, Uzi, and many many many more. I shot all of them, except the AR-18. I never bought any FA that wasn't a pistol caliber at Jerry's encouragement. I was really tempted to buy a Colt 603 from him, but he discouraged it heavily. When I sold all of my Class 3 guns (last one sold in 2001 IIRC) I watched the internet market carefully and SMGs sold much better than non-SMGs.
Sorry for the derail, but there were comments about the AR-18 in this thread.