Always sight your slug gun to be roughly 2.5" high at 50 yards, at 100yards I'm getting clover leafs. Once more shotguns are like women, you never know what will make them happy, so you need to try various slugs. Try any of the major saboted slugs out there, Hornady, Remingtom, Winchester, Federal, Lightfields, etc. the latter seems to always be excellent or at least decent out of most shotguns. Also stay with 2-3/4" shells, no need to go with 3"
A few years ago I brought myself a Remington 11-87 SPS at first I sighted in with Lightfields EXP 2-3/4" with great results, few years latter I was hunting Illinois and wanted a lighter slug I can reach out further, I got decent results from all the majot brands and settled with Winchester's Partion Golds, well yesterday I went back to the Lightfields, and with very little adjustments I was back to my great groups