C13OE
Well, the AGI one looks more comprehensive, but generally I found them to be so superficial that they are of limited benefit.
I enrolled in a crappy one and only finished it because I started it. If there is a beach of gunsmithing knowledge then I only have one grain of it so far, and got that knowledge from the books that I collect and read, from the forums that I haunt (there is a ton of knowledge out there!), and from getting my hands on every firearm that I can and cleaning/fixing/etc. to help increase my knowledge and experience.
As for machining, look for the Machinery Handbook, although Dunlop does discuss a bit of it in his book "Gunsmithing."