Well the Rem derringer was made from after the civil war till up into the 30's maybe? So if it were some kind of 19th century percussion gun the ffl part may not apply, a gray area for me. But since it is a cartridge firing arm that might be 20th century, I would still go with a gunsmith. The rules might not apply either since the gun is a curio relic and the ammo is not readily available but why split hairs, a non gunsmith might plate it but unintentionally turn it into a paperweight. YOu can buy stuff from Brownells to plate your own guns, but on a first try, the results might be unappealing. Gunsmith is safest.