Say it with me..... "I don't know"
"Nothing" is a rather far cry from "I don't know". Those two are not synonymous - I can know that it means nothing, even if you are still unconvinced and want to believe that it was placed there to somehow actually modify the term "firearm" which precedes it. In reality, it does not. It does nothing and has no meaning, as are a great number of clauses commonly found in modern legislation.
Can it have a purpose without a meaning? Sure, even as it does nothing.
I'd also like to stress at this point that it could not have any less meaning than it currently does in the context of the sentence containing it. This is as close to a mathematical proof as I will be able to present on the matter. If it is not sufficient, perhaps I could come up with some diagrams or pictures.