12 gauge is fine. Since I reload I load 1 oz of hard #5 shot and use it both for rabbits and squirrel. I figure with one ounce of a large shot size there arent as many pieces of shot to worry about. By the way I pretty much use my 1 oz trap load for this. Also, I use a fixed full choke barrel all the time on my old Winchester 1200, occasionally I'll put the modified on but 90% of the time its full choke and aim for the head or nose, doesn't mess up a squirrel or rabbit like some may think.