Hmmm...
Missoula passed its anti-gun ordinances long before any sort of preëmption bill was even introduced into the Montana legislature.
Therefore, it might be argued, depending upon the statute's actual wording, that Missoula's anti-gun ordinances take precedence over the provisions of the state-law preëmption statute, and therefore are neither superseded nor nullified by it.
My guess is that it may have to be fought over in court.