Just last week I sent a gun barrel to a smith in MA from a small town PO in IN and insured it for $400. They asked if there was an hazardous material in the cardboard tube but never asked about a gun and I didn't volunteer any info. Tracking was spot on.
Early this year I bought a watch on e*ay and the seller sent it priority mail. It took about 3 weeks to get to me and tracking was of no help. When it arrived finally I saw that it was in a plain cardboard box with priority mail written on the side in pen.
LH