In my experience with SXS Paradox guns, increasing the load will move the groups apart, and a decrease will make them come together. I think it has to do with recoil movement, and realize this is opposite to what we expect with a double rifle. I have used Unique, around 22.5 gr. with success. Can't speak to the charge of Herco. This loading at the advise of Ross S. with a 750 gr Fosbery bullet and hard waxed wads. We are talking about small powder charge changes to move the groups here- sometimes a 1/2 gr change will give noticeable group movement.

I have no experience with O/U guns, but Graham Wright says the basic principles apply.

Enjoy! Doug