The frame/reciever is the gun, legally. It's the serial numbered part. All other gun parts are just parts, and do not have to go to dealers, and can be mailed or sent via any carrier, with no restrictions.
Barrel, cylinder, slide, magazine, hammer, springs, etc, are just metal shapes. No regulations (other than state laws about magazines, which may apply). Frame/receiver (with parts attached or not), its a gun, and both Fed and state laws apply.
