I used(still am) a weatherby grade III Athena 12 gauge over under, with a modified and improved cylinder choke. Running federal #5 shot. They're the ones with the pheasant forever logo on the box.....
But, I also run my 28 gauge . Stocked with # 6 shot with equal success...
Are you gonna be shooting wild birds, or set? Any shotgun you shoot well will prove lethal. Best of luck to you on your upcoming hunt!