"Derivatives of primary functions"? Not sure exactly what you mean by this. Is a kid's cap gun a 'derivative' of a .44 Magnum? In one sense it is, but it's absurd to suggest that it's therefore a weapon...
Hitting someone over the head with a match air rifle is likely to cause much more harm than shooting a pellet from it.
Does this make it a weapon? It just means that like any other heavy object it can be (mis)used as one.
Obviously you have a point about safe handling, and airguns should be treated as dangerous from a safety point of view, like any gun.
But are airguns 'weapons'? No, they are not.