No transfer required it's not considered a firearm.
Well, sort of...
If the person you're transferring the weapon to has any provision that does not allow them to have possession of a firearm, then you're both in trouble should you get caught. Even a muzzle loader is a firearm when applied to them. A partial list of ownership limiting issues:
State law forbids many individuals from possessing firearms including:
Those previously convicted or found not guilty by reason of insanity of any felony.
Those previously convicted any of the following offenses if it was committed upon a family or household member (domestic violence).
Assault in the 4th Degree
Coercion, Stalking or Reckless Endangerment
Criminal Trespass in the 1st Degree
Violation of a No Contact Order or Restraining Order.
Currently under a domestic restraining order
Having been previously involuntarily committed for mental health treatment.
Pending trial, appeal or sentencing for any serious offense as defined by RCW 9.41.045
Federal law also prohibits firearm possession by any individual who:
Is under indictment in any court for a crime punishable by imprisonment of a term exceeding one year. (Arrested but not yet convicted).
Anyone convicted of a crime punishable by a prison term of at least one year.
A fugitive from justice.
An unlawful use of or addicted to any controlled substance including marijuana, medicinal or legalized by your state.
Anyone adjudicated as mentally defective or has been committed to any mental institution.
An illegal alien.
Anyone discharged from the military under dishonorable conditions.
Anyone who has renounced their United States Citizenship.
Anyone subject to a court order restraining the person form harassing, stalking or threatening an intimate partner, or child of the intimate partner.
Anyone convicted of a misdemeanor domestic violence offense.