In MOST states BP firearms are not considered to be "deadly weapons" under the meaning of law.
Also, "homebrewed" firearms need NOT be serial-numbered according to BATFE, though it's certainly a good idea for "recovery" if it's lost/stolen.
Some local/state jurisdictions require all firearms to be serial numbered and will, if asked, "assign" a number to your "self-built" receivers.
(We found this out a couple of years ago, a couple of weeks, before an "AK building clinic", which was held in Richmond/Henrico County, VA.)
yours, satx