Checked the scope seems ok, not dropped everything tight,worked ok on my 12ga last year. I will try the different loads you listed. Why back cap off two turns? I,ve always cranked it tight.
The way it was explained to me as a young lad, was that some slug gun barrels must be able to move ever so slightly as they discharge. Think of cracking a whip. Screwing them down tight prevents this and can actually effect accuracy. Just a notch or two, no more. Those of us with lots of gray hair in slug country remember all the tinkering that we had to do in order to squeeze all the accuracy out of the slug technology of the day.