What has a library to do with the political subdivision (city, town, village) in which it is sited? The local library here is a branch of a county library. I may add more on later edit ...
This may help http://dpi.wi.gov/pld/trusteefaq.html
None of the library policies, collected here http://dpi.wi.gov/pld/policies.html, addressed firearms/guns.
The governing statute chapter is Wisconsin Statutes Chapter 43 Libraries