Flintlock and percussion muzzleloading rifles, musket, and guns are not c&r but they do not require any ffl license to ship or receive. They are exempt. Just box it up and send it.
In addition, a Class 03 Collector of Curio and Relic licensee (C&R) would not be allowed under law to receive a C&R firearm for another person. this would be acting as a dealer which is prohibited for C&R license holders. Anything that would have to be sent to a FFL holder would have to be sent to a dealer