I tried the gunsmith by mail from an outfit in AZ. Was basicly an open book test, that dint tell you a lot. I had been out of the service for a couple yrs so used part of my G.I. Bill to pay for it. I dont know about the other mail courses but in my opinion I think I already knew more than what the course offered.
As gunnut said. get yourself a lot of books on the subj. and study. Thats what I wound up doing. I learned a lot of things by trial and error, but I had a bunch of junk guns I had picked up to pratice on. I am not a gunsmith, nor claim to be one, but I like to tinker with my own guns, rather than have to pay to have it done. Something major and it goes to my gunsmith.
Gun Runner