Sounds like he manufactured an illegal weapon over 50 cal.
He manufactured an illegal weapon, period. It needs to go through a FFL. It is a ghost gun in Wash state, they are illegal.
"A black powder firearm is regulated under Washington State Law and requires all state mandated transfer requirements (page 9 WA State DOL Firearms Dealer Training Manual) and under Washington State Law, if you are prohibited from owning or possessing a modern firearm, you are prohibited from owning a black powder firearm as well"