ME- My 20 ga. won't hold a good pattern at 40 yards. I feel that the copper plated # 7 1/2's would give good penetration at that distance but the pattern gets too thin. I am only shooting an ounce and an eighth of shot. Now if you were to go to a 3 inch shell and an extra full choke you MIGHT get a 20 ga. to pattern out to 40 yards. Which ever gauge and choke you decide to try be sure to pattern several shells ( loads, shot sizes, brands,) on a turkey head patterning target. I want to have six or more pellets strike a vital zone (skull and vertebrae ). 30 yards is the max for my 20 ga. Having said all that, I've only killed one turkey with my big 12 ga. that I could not have taken with the 20 ga. It was killed at nearly 50 yards. It was the second bird of a double that ran instead of flying. The load was Federals three inch magnum, 2 oz. copper plated #6 's. That is a swarm of pellets coming out of an extra full choke tube.
