Actually, pretty sure serial and licensing isn't necessary-ish unless it's an automatic, a sawn-off, greater than .50 caliber or to be sold/transferred.

Which make ye wonder how BATF can determine when an unlicensed homebuild was built in someone else's home, and if it was done by the owner in someone else's home would count as joint-custody? (one could claim they track parts, but you can scratch-build entire guns with an illiterate 19th century French-maid's education and an anvil [lol. RSC mle. 1918])

Pretty sure 89% of the time we just refuse to worry about it until someone stumbles upon an unlicensed munitions-factory in someone's garage/basement.