I’ve mailed rifles in corrugated cardboard several times without any issues. Often the postal people do want to discuss firearms under those circumstances, even in plain cartons however. Last fall I made a simple crate from 1x4s and 1/4” plywood. It didn’t look like the usual ‘gun box’. Then I stapled cardboard to all the edges (to cover rough corners). The counter folks looked like at it like it was full of table legs or something, not a peep or skeptical glance. The only way to open it was to remove screws; pretty secure. I’ll certainly be doing that again.
