I have come into possesion of a Harrington and Richardson 12 ga. single shot, model Topper 88. It appears to have been used as a 'truck gun' as it is a bit of a beater. The bore looks unfired, though. There is some fairly deep surface rust on the top of the barrel near the breech, as well as on the frame top between the breech closure and trigger. How do I get rid of this rust? Can I fire 2 3/4 in. shells in a 3 in. gun? Thanks.