In Montana..., Local Governments can Regulate Firearms in their own: 1. Public Buildings, 2. Parks, and 3. Public Assemblies.
However, they can not Regulate Firearms in any other manner, so as long as you are not Prohibited form Owning, Possessing, Carrying, Transfering, or Transporting them in accordance with Federal and State Law.
Open Carry is Legal in Montana, and Concealed Carry is Legal in UnIncorporated Areas, beyond: Cities, Towns, Logging Mine Camps, etc..
Open Carry can not be Infringed in Montana by Local Governments which Enact the provisions as stated in Sentence 1 of this Thread, BUT they can Prohibit you from entering those Locations with a Firearm at all.
Most, but not all, of Montanas' Jurisdictions have chosen NOT to Enact those Optional Off-Limit Locations.
Please read Montana Law 45-8-351 AND Montana Law 45-8-316.