If I were you I would ditch the 3.5" cup especially if you only plan to shoot it say half full of hevi-shot. They do make them in smaller sizes or you could experiment and cut the cups down to the size you need to hold the amount you would like to shoot. The wad fillers could be the issue. You are using good stuff to power the beast so I can only think that the fillers upon ignition may be shifting in the cup toa point in which the shot is getting upset. I fill an entire wad and overcard and ram it tight so I think when I get ignition it like a slug shooting out. Have you ever retreived your wad? I usually find them a ways down range but they look similar to a 50 cal sabot all splayed open. The wad is never burned on the bottom either, it may have a dust colornever burned. That's telling me there was nothing hanging up inside and ignition was smooth. In 50 cal, sometimes you hear of sabots getting torn or split in half downrange which is not what you want of course. Check the inside of the barrel and make sure it is smooth as silk after cleaning. I usually get an alcohol patch run it through then get a dry patch and run it to see if any fibers hang up inside. If they do I reclean it with a nylon brush and repeat.

The other thing I think it could be is you outside choke tube.I use the one that came with it and if I remember right they do make one for steel and hard shot which I beleive you can get from Knight by calling them. I remember because there were 3 diffrent ones I was going to try when I got mine but I wanted to try the one that came with it first and seen no need for the others asI shoot lead shot. I think the manual has some info in there as well. Hang in there, that is one helluva a gun. Again I would look and shortening the wad cup and even look at a diffrent choke tube. I am heading out tomorrow morning to get that big ol tom that keeps hanging up in the fields from me. Hope to connect.