I recently bought a stripped AR lower from a gun store, and brought it home to show my stepfather. We were looking it over and he noted that the serial number had some personal significance to him, and he said he might be interested in building his own AR when I do mine. I told him that if he wanted to build one, he should buy the same lower I got and we could trade them so he can have the serial he'd prefer. Do I have to do anything as far as paperwork to trade these 2 like items with different serial numbers or can we just swap them and fill out the MA forms when the rifles are built? I think this is a pretty simple thing, but I haven't sold/traded/transferred any guns until now.