IANAL, but during a political debate I heard on the radio (I think it was 620, IIRC), the property laws regarding posting political signs were touched on because of somebody posting a sign the city ordered him to take down.
From what I remember, your property extends to the sidewalk. The stretch between the sidewalk in the street is your responsibility to care (shovel snow, cut grass, etc), but is considered public property, therefore you have no rights to post political signs there, and I would imagine this translates also to guns.
Just my 2 cents...