Running the numbers through KPY Ballistics, it looks like 5s are probably going to be your best choice. Sixes have the penetration to kill to 37 yards, 5s to 47, and 4s to 57 when launched at 1200 fps. Sixes get the edge in pattern density as one would expect but are limited with regards to penetration. Fours win hands down on penetration but are going to run out of pattern way before that. I would pattern both 5s and 6s and see how far you can keep 100 pellets in a 10" circle. That is your max range, with the caveat that 6s are out of energy at 37 yards. Good luck!
