I'm a Texan living in Sweden. Last summer I took the kids to see the family in Houston. My brother bought my son his first BB gun, a classic Red Ryder, which he loved of course. I did all my homework to see what I needed to do to get the BB gun home. I checked United and TSA's online documentation, which indicated I could simply check it with my bags. Just to be on the safe side, however, I put it in a hard-shell case with a lock on it, since we would be landing in Denmark, which not only has very strict gun laws, but strict BB gun laws too (fgts).
As we were checking the bags, the United agent points and asks what's in the case. I said, "it's just a BB gun." Then she got that look on her face and I knew it was going to be trouble. She got on the phone and her walkie-talkie and talked back and forth with some people for several minutes, and then she said, "You can't check that bag. You're not allowed to transport firearms through Denmark, and United Airlines considers a BB gun a firearm." I said that a BB gun is not a firearm and pointed out that even the TSA makes the distinction between BB guns and firearms on their web site, but none of that mattered to the United Airlines agent; they just make up their own definitions of words, and would not let us check the bag.
My son held back the tears, but he was really upset. I'll never fly United again.