The big magnum 12's with tight chokes will give you some extra range but not as much as you might think. Lesser loads/gauges will kill turkeys every bit as dead if you are content to get a bit closer before pulling the trigger. I used to shoot heavy loads in a 3" 12ga and killed turkeys out as far as 52 yards with it. I switched to a 3" 20ga about 10 years ago. It kills turkeys every bit as dead and only gives up about 10 yards in effective range.
