I've had great luck with Brenneke KO Sabots (not the standard KO, but the one with a sabot). They are a 63 caliber slug, 1 oz at 1650 fps. Shot a pile of deer with them and they perform real well. They are relatively cheap as sabot slugs go. Your 500 may like them, or not. Just have to try a few.
I've seen improvements in accuracy from tightening the barrel nut. Be sure yours is tight and stays consistently tight was you shoot and test slugs.