I built a pair of ARs from a pair of FFL registered receivers so I could learn about how every part works and to pick every part. Once looking at the receivers, I longed for the days when I had a fully equipped machine shop because a receiver made from bar stock would be unique. Then I longed for a nice shop to work composite materials because I speculated that making a mold for a receiver to make a carbon/composite receiver would be even easier. Why? To learn if I could make something I was interested in. I like making things.
Considering how many parts Glock fanboys can switch out for fun, there isn’t much left that is stock for the hardcore parts swapper. An 80% receiver and jig pretty much guarantee than anyone competent with tools can get a functional receiver with a small shop in a reasonable amount of time. With more skill, better tools and more time one could do this from scratch. If you have a small club, sharing the expense of the jig, tools and learning curve might make for an enjoyable winter project.
The real joke is the 80% Sig P320 receiver. I bet I could finish one with a hand drill and a set of files from Harbor Freight.
I am very sure that selling your finished receiver is a huge felony. Giving it away might be a problem and personally I would scrap any receiver I made before risking any violation.
I am pretty sure that there is a federal tax on completed firearms that manufactures must pay. A long time ago I asked a manufacturer why I could buy a completed upper and completed lower for less than the cost of the identical similar model and he said there was a tax and while I could of course complete the rifle without issue, I should not sell the completed rifle or technically I would owe tax. Yes, common sense tells you it’s one pin...
But then how the heck is that little piece of bent metal a sig 320 receiver? It would be more complicated to make an office binder clip than that thing!
On the other side of the argument, I am thinking of an idiot kid I know that not only lost his rights to own a firearm but proved to me he’s incompetent to own one... yeah, that idiot has friends that are dumb enough to make one for him.