I hadn't even thought of this before, I would call any long gun a rifle whether it's barrel is rifled or not. Excellent point.
There's been a recent thread on another forum about airguns being weapons which got quite involved (I should have contributed but I didn't).
My opinion is similar to yours and whilst in law they considered "air weapons", my view of a weapon is something that is designed to kill or injure another person and I can't say that an airgun is, though it can be used as one. I would also say a hunting rifle isn't a weapon as it's specifically designed to hunt quarry whereas an AK47 is designed to be used in armed conflict so that is a weapon.
I think a lot of things can be used as weapons even though they're designed for something else.