I must be doing something wrong, I've never yet been asked what's in a parcel.

TBH, if its not a prohibited item, as far as I'm concerned its none of their business exactly what it is. Rather than have a discussion about carrying airguns I'd probably just lie. Maybe I'm a bad man but life's too short.

(Just to avoid confusion, I am NOT saying I would try to send a prohibited item by lying about it. Just that I can't be bothered explaining their own rules to them)