I don't know that there is a best range, but for conventional copper plated lead shot I've heard the rule of thumb is 40 yards maximum for shot size 4 or 5, and 35 yard max for size 6 shot. Now, let me say that I wouldn't recommend you shoot at those ranges without first patterning your gun and ensuring that your set-up can put a significant number of pellets in the kill zone at those ranges. Personally, I like to get a turkey within 20 yards to take him. I usually put my decoys between 15 and 20 yards to try to bring him into that range. I have no experience with heavy shot, and the folks that do may indicate that the effective range for that is somewhat longer.