Your 12 bore is fine. The loads you're looking for are called field loads, special field loads, or heavy field loads depending on the manufacturer. These have a muzzle velocity of about 1200 fps, with an ounce and an eighth or ounce and a quarter of shot. The shells are typically low brass like trap and skeet loads. I like #4 shot, but others prefer #6 for better pattern density. You might find that a modified or cylinder choke prevents the game from being torn up as badly as it would be from a full choke gun. Try several brands to see which patterns best your your gun. I like short barrels myself, and the only advantage of a longer barrel is a longer sighting plane, and in some cases a better balanced gun. Anything from 20"-26" is appropriate, and 18" works in a pinch.