I would have an attorney draft a letter of request/demand to be made whole and have it sent certified to the Chief and the entire City Council. I would think the letter should have a description of the case, case #, serial # of the firearm and full description and replacement cost of the items missing and final judgement and whatever else that the attorney would deem necessary.
Some may think "Governmental Immunity" in this case but I doubt it-GI only covers the entity in the "Good Faith" executions of City duties. It will not cover them in the case of employee negligence or theft.
I would think that reasonable timeframe would be 30 days to be made whole.
If they balk then contacting the BATF may be a good hole card to have in your hand but I would not tip your hand just yet. I doubt the BATF would show much humor with missing firearms especially from a PD evidence locker. I would think that anybody with access to said locker during the time in question would get pretty nervous if the BATF started poking around.
I would definitly contact an attorney though before contacting the city.