I wouldn't expect more than about 60 pellets in a 10" circle at 40 yds, not enough to ethically shoot. You are probably looking at a 30 yd gun with those shells. Whatever range your pellet count drops below 100/10" is your max range. Fives have plenty of energy to kill way beyond where your pattern is going to get too thin. There are only 212 pellets in a 1.25 oz load of 5s. For that reason, you will extend your range by going to 6s to get your pattern density up. They will still have more energy than needed to cleanly kill a turkey beyond the range your pattern runs out, but that range should be further than with 5s. A 1.25 ox load of 6s has 281 pellets. I personally wouldn't hesitate to shoot 7.5s in a 20 ga. They have the energy to kill to 35 yds, and 1.25 oz of 7.5s contains 437 pellets.
