I have a 512 Slugmaster. My advice to you is to first make sure there is no lead fouling in your bore. You will have some if you have been shooting rifled slugs or hybrid style slugs (some lightfields, some brennekes, Remington Buckhammers) as opposed to sabot slugs. So clean the bore properly.

My gun seems to prefer sabot slugs with velocity less than 1700 fps. I am shooting Brenneke KO Sabot slugs now and they are awesome. I've never had a deer take a step. I've also used Winchester BRI-style sabots and they shoot real well. Tried Hornady H2K and they were all over the place.