I have one on a 870 super mag. that I use for turkeys. I put a large poster paper out at 40 yds., patterned the gun, and adjusted the red dot scope like any other scope so that the pattern was centered. Probably could do the same at 20 or 30 yds., but I figured that 40 was the max. I would want to shoot and wanted to see how dense the pattern was at that range.