Sounds like your gun is underachieving with the Hevi Shot for some reason.
I tweaked my Mossberg 535 tactical with an Indian Creek Choke and shoot 3 1/2" 2 1/4oz. Hevi Blend (5,6,7s). I wasn't able to accomplish the 230-240 pellet count at 40yds in a 10" circle like some but I am at the 200 count. I can tell you that it absolutely leveled my bird at 45 yards this year. I only had 9 pellets low in the body when I butchered him.