Has it ever occurred to you that anyone with basic machine shop skills
and tools can make a decent gun? If you start making guns difficult
to buy, it's not hard to imagine a large underground cottage industry
starting up - very similar to what happens with illegal drugs. Are
you also going to regulate lathes, milling machines and grinders?
The world is awash in totally unregulated "parts" too so you don't
need to make the whole gun. There are guys selling AR lowers that are
9x% complete (still just a chunk of metal) Then you drill a couple
holes, grind out a spot or two and buy a "parts kit" for the rest.
To be legal you get a BATF form 1 ... or not.