Hey Ironcreations, I don't call myself a gunsmith anymore but, I took my tool and die makers apprenticeship from a guy named Klause Horstkemp ( now deceased but Google him anyway and see what you get...)
He was the owner of State Arms Gun Co and basicly invented the first sucsessful single shot rifle to use the .50 bmg round.
We provided rifles to the military and police agencies as well as to serious western game hunters. We also made .50 BMG hand loads that used a solid bronze bullet turned on a cnc lathe. Wickedly acurate but, kind of pricey at $5.00/ea. (that was back in the 80's)
Man the stories I could tell about that guy...
While I worked there I fell under his Firearms Manufacturer licence and was able to do any thing in the way of gun and "accessory" making that I wanted to as long as it was cleared by him and had the correct paper work filled out if for sale.
At one time, befor the ban, a well known class three dealer offerd to sell me a nearly new never been fired Maramont M60 with spare barrel and everything (no tri-pod though) for $6000.
I couldn't come up with the money and had to let it pass but, I just saw one of those guns for sale not to long ago for $47,000 !! Ohh well...
Those sure were good times working for him. I sure do miss him.

back in the 80's I was more worried about building jeeps, I wish I knew what a NFA item was, I would have some much cooler stuff now, and might be typing this from the recliner instead of the shop where I have to still work all the time. People tell me how they bought stuff for next to nothing and just sold it for 20 grand. like you said, oh well.. I have a 50 M2HB now I am building, barrels unlined go for about $500.00 to $700.00 for a M2, the last one I have is in the $600 range, cant remember for sure the exact price. it matters on condition, the one I have gauges about a 2. the scale being 1 to 20, 1 the best 20 "the xxxx near dumpster material". I also sell some of the 50 parts, like the left sideplate, The RSG assembly, and a few others.