You could, but it would be a little odd...
It would be like having a glass of water, and saying "This liquid is delicious". Perfectly valid sentence, but people would look at you funny.
Confectionery is a group term, and would usually not be used for a specific piece that you are pointing out.
As for a sentence as "This firearm is rare contraband", would be a normal sentence, even if contraband (and firearm for that matter) are both collections. Contraband shares a common property, being illigal, and that is what is being refered to.
As for why firearm is acceptable is because further precision would not be needed, and only complicate things. It doesn't matter if it's an mp5, dessert eagle or old western revolver, they would all be put in the category of contraband.
Sadly none of this is a precise science, and relies a bunch of common knowledge, about what is common knowledge. So the challenge is to understand what your recipient knows, and set your wording accordingly