I have a Marlin bolt action 12ga that has a 36" full choke barrel. It will not hurt your gun to shoot buckshot. But the choke will deform the buckshot pellets. You will not get good patterns. For a truck gun or a gun in a shop it will be fine with buckshot.
Spray your trigger assembly with WD40 and dry fire the gun or just take out shooting. WD40 is not the best gun lube but it is great for removing rust. WD40 will also cut gummed up oil and grease. If WD40 will cut 70 year old cosmoline it will cut any gun lube.