I also have the Marlin 512 Slugmaster.

It shoots Win BRI style very well. This year I switched to Brenneke 2.75" KO Sabots (a true sabot, not their hybrid style) because the Winchesters now cost over $11 a box. The Brennekes shoot real well too, and cost $8 a box.

I tried Hornady SST and they did not group well at all for me. I only bought one box, and maybe it was a bad shooting day, but with the cost of most of the fast slugs, I'll stick with the Brennekes.