FrankenMauser, were they trying to bring them home via either checked bags or carry on? I could understand that, but sending them through USPS?
General (military) Air Cargo. (A perk of our unit - unavailable to most service members.)
And, even as a (legal) portion of a shipment of helicopter parts.
The only ones I ever saw make it back to the states, were the ones that were truly smuggled
back - under helicopter floors, inside engine casings, between layers of vehicle armor, inside damaged rotor blades, etc. (I am not suggesting or advocating doing such.) And all of those guys still got caught or turned in, anyway.
If I were in your position, I would try shipping a single rifle back.
If it fails, you can resell it without taking a huge loss.
If it works, you can buy more and try your luck again.