Comment by rolph

personal/private manufactured firearms, are not required to be serialized, unless they are transferred to someone else, and that needs an FFL for the transfer and the stamping of the number [part of the FFLs license number is L holder to attempt to make a valid serial number, however "identifying" markings are highly encouraged [not reqd].

and of course there must be no legal disqualification for ownership/possesion of a firearm, and the reqd paerwork for short barrel rifle or automatic weapon must be possessed if you intend to build one , [note "new" automatics are banned from production unless you are FFL6 or FFL10 and likely a special operator/armorer under govt contract]