I am interested in building several guns for myself. I am thinking of machining all parts myself, NOT buying an action and assembling the pieces. Can I legally do it?
Only with a Federal Firearm Maufacturer's license. Unless you savor having JBTs kicking in your door in the middle of the night looking for an excuse to shoot you, I would recommend not doing this unless you either get a license or purchase a legal, serial-numbered, receiver or frame.