Although it is Illegal to Carry a Concealed Firearm in a Bank in Montana, per Montana Law 45-8-328(b), unless the area is one that either 45-8-328(b)(i) or 45-8-328(b)(ii) covers, the Law ONLY applies to Concealed Firearms.
Montana is unique in this aspect, because; Montana Law only disauthorizes Concealed Carry in 3 specific places, the other two are: 1. Restricted Areas of Public Builidngs AND 2. Bars/Restaurants where Alcohol is served for Consumption-on-Premesis.
Open Carry in all three aforementioned locations is Legal.
Montana Law 45-8-328(2) bars any Defense under this Law as to whether or not a Person has a Concealed Carry Permit. This Crime is Punishable, per Montana Law, with a Fine of up to $500 and up to 6 Months in Jail.
It is also Illegal to Carry a Concealed, but Concealed ONLY, Firearm while Under the Influence of an Intoxicating Substance, 45-8-327. Again, as before, Open Carry, whilst, Intoxicated, is not Illegal, under Montana Law, nor is being Intoxicated in Public.