jeff,
another shell that i forgot to add is the winchester high velocity 3" 1 5/16oz #5 shell, between this shell and the federal flight control, they are probably the 2 best performing 20 gauge lead shells on the market, either one or both of these shell will give you killing patterns at your desired distance.
the best way to test is to get a 30"x30" piece of paper and shoot at it from your desired distance, and then draw a 10" circle around the densest part of the pattern and count how many pellets you can get in a 10" circle, if you can get at least 75 pellets in the circle then you have a killing pattern, the distance at which you can no longer get more than 75 pellets in the circle is your maximum effective range.
good luck Zack, and if you have any further questions please feel free to pm me