Mark
I'd assume (dangerous I know) that when you declare "it's an air pistol" the post office should know to send it via the appropriate transport. I'm not saying the doomsday "shipping off to Belfast for examination then destruction" scenario can't happen, but I suspect when it has it's been when the contents have been declared as something else vague and innocuous.
“We are too much accustomed to attribute to a single cause that which is the product of several, and the majority of our controversies come from that.” - Marcus Aurelius