If you have not already, try using some wadding, either a trimmed 12/10grade plastic shotgun, or hard cardboard. IT did the trick for me. See double forum or felt wad post for results! I'm pretty pleased!

It’s been a battle royal with this gun!

I believe the reason this helped is that I was getting a delta in pressures per barrel. This seemed to have helped with this. As well, I use less lube, and the wad keeps the powder from contacting the wet patch, rendering less powder inert.

Just a theory.

Colorado