If was a rifle or shotgun, you can send it US mail without a problem. If it was a handgun, different situation.
Consult the (Domestic Mailing Manual ) DMM 601.1.3 at :
http://pe.usps.gov/cpim/ftp/manuals/dmm300/full/mailingStandards.pdffor clarification.
11.3 Rifles and Shotguns
Although unloaded rifles and shotguns not precluded by 11.1.1e and 11.1.2 are
mailable, mailers must comply with the Gun Control Act of 1968, Public Law
90-618, 18 USC 921, et seq., and the rules and regulations promulgated
thereunder, 27 CFR 178, 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
gun is unloaded and not precluded by 11.1.1e.
The referenced sections do point out what does not qualify. The US Post Office is pretty casual about this as long as they are packed correctly and you are willing if asked to open it or sign a statement that the weapon is unloaded.
11.1.2 Handguns
Pistols, revolvers, and other firearms capable of being concealed on the person
(referred to as handguns) are nonmailable unless mailed between the parties listed in
11.1.3 and 11.1.5 after the filing of an affidavit or statement required by 11.1.4 and
11.1.6.
11.1.3 - 11.1.6 do not apply to what you are suggesting. This the USPS Bible. Heed it well
They will mail anywheree in the U.S. by this rule