This has come up in the past because many of us mail our long guns to ourselves, gunsmiths, and manufacturers for repair. Here is the link to the applicable page of the Domestic Mailing Manual DMM 601. The main section we are interested in is 11.3 Rifles and Shotguns but be sure you read the other cited paragraphs i.e 11.1.1e, 11.1.2. Also see 11.4 and 11.6 Not every postal employee is familiar with these regs and NONE of them are authorized to make a legal judgement as to whether your gun is mailable.


http://pe.usps.com/text/dmm300/601.htm#wp1065405
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Mean As Nails