I don't understand why people argue about things like this when the law and regulations are clear and easily obtainable.
432.3 Rifles and Shotguns
Except under 431.2, unloaded rifles and shotguns are mailable. Mailers must comply with the rules and regulations under 27 CFR, Part 478, as well as state and local laws. The mailer may be required by the USPS to establish, by opening the parcel or by written certification, that the rifle or shotgun is unloaded and not ineligible for mailing. The following conditions also apply:
Subject to state, territory, or district regulations, rifles and shotguns may be mailed without restriction when intended for delivery within the same state of mailing. These items must:
Bear a “Return Service Requested” endorsement.
Be mailed using a class of mail, product, or Extra Service that provides tracking and signature capture at delivery.
A rifle or shotgun owned by a non-FFL may be mailed outside the owner‘s state of residence by the owner to himself or herself, in care of another person in the other state where he or she intends to hunt or engage in any other lawful activity. These mailpieces must:
Be addressed to the owner.
Include the “in the care of” endorsement immediately preceding the name of the applicable temporary custodian.
Be opened by the rifle or shotgun owner only.
Be mailed using a class of mail, product, or Extra Service that provides tracking and signature capture at delivery.
https://pe.usps.com/text/pub52/pub52c4_009.htm