In the current climate of gun = evil, it's best if nobody KNOWS there's a gun in there.
They can suspect all they want, based on size and shape. But it truly could be golf clubs or two baseball bats or etc etc etc.
Telling them it's 100% a gun, or labeling it as such, is asking for some gun-hater to stomp on it or throw it or bounce it down some stairs.
With insurance you're simply stating the value of the package, not making a claim about what's inside.
"Sporting goods" isn't a lie. There are golf clubs that cost more than our airguns, and would be shipped in a long cardboard box. Sporting goods is an appropriate descriptor and would apply to golf clubs, or airguns. Makes sense that somebody would insure expensive golf clubs, same as an expensive airgun.
Package it good enough to get there safely and don't rely on insurance to bail you out. As for getting "lost" and needing insurance then, I'm of the opinion that a package labeled "sporting goods" is more likely to get to its new home than anything saying "gun".