Halcon,
I don't know if things have changed in the last year or so, but I have mailed a shotgun in two seperate boxes without issue. I pulled a shotgun apart, boxed up the stock and action in one package, then the barrel and forearm in another. Without the barrel, it is inoperable as it is just parts. I don't know if I was suppose to do it that way, but the USPS didn't bat an eye.