When shipping a long gun in a cardboard box for sale, I go to my friend's mfg shop's shipping dept and I wrap in many layers of a commercial foam sheet wrapping mat'l, up to a couple inches or more thick. Then make a box the correct size from a larger box by cutting it down. I then take sheet cardboard and line all sides inside the box with at least 2 or 3 addition layers to protect from sharp penetration. Then I put the foam wrapped gun in and pack all around with additional foam wrap until it's tightly fitted. Sounds like overkill? Well maybe. But I don't think it is.
Most people don't know this, but all cardboard box mat'l is not anywhere near the same strength. Look at the boxes for Chinese products and then compare to a 220 psf rated new box made in the US. Big difference.
If I'm shipping to a stockmaker or gunsmith and expect the gun back, I put it in a inexpensive hard guncase first then pack it up in a cutdown box fitted with multi layers of sheet cardboard for protection.
Although I know a lot of dealers and others do it and get away with it most of the time, I would never ship in a single layer cardboard box, never.
