They are a private company taking responsibility for transporting & delivering your item, so they are 100% entitled to know what they are carrying,
If you don't like their terms...don't use their service.
Apart from that it's pointless to lie or misrepresent your item, it's legal to send a low power airgun, or parts of one & if you need to claim the compensation you're still going to have to prove what it was & what it was worth, at which point the rules for a gun will apply.