An antique firearm is considered a "firearm" as is a BP replica. The defining criteria is "projectile launched by burning propellant." -- words to that effect.
I was thinking BP guns were not subject to CCW laws. They are.
My understanding of the USPS is that NO firearms are allowed for mailing/shipping. It's not lawful to bring a firearm (and that means a receiver), onto post office premises.
You can ship gun parts, barrels, grips, hammers, bolts, sights, stocks, magazines. No receivers. My understanding is that even FFL cannot use USPS to ship firearms. You use FexEx or UPS.