Your best bet is to find a MP5 forum and research before you build.You cannot make it F/A or selective fire.So theres mods you need to do to the FCG to be legal.Also it can explane the whole SBS/Rifle thing.Good luck and post pics of the compleated gun.Happy building.
im not planing to sell it so im just gona build the damn thing and pin the barral in place and when the time comes for when i want to regester it as a sbr i will take it from there