Here's some history trivia on the Watson upper.
It was designed and marketed by Dan and Jen Franklund in 1999. They would sell you the barrel extension and the bolt for $150 or so and with a $150 surplus Ma Deuce barrel, you could assemble it into a 50BMG upper. The parts came with a serialized binder full of detailed instructions as well as photographs and blue prints of all the parts. I have number 8, dated June 6, 1999. The Company was called D&J's Habitual Tinkering in Bismarck, North Dakota.
I actually never received the bolt, only the information with the barrel extension. The bolt was permenantly back ordered and never arrived. I took the barrel extension, which is actually the receiver itself in this application, and built it into the Tromix 50BMG over-the-shoulder LAWS rocket style rifle which I sold a couple of years later for $400. Even at that price, the thing was for sale about 6 months before it sold.
The Watson unit works fine, but it is a pain in the ass to break the gun open and unscrew the bolt after every shot.
Tony Rumore
Tromix Corp