Have you tried Lightfield slugs? I love them and my gun shoots them well, and from what I have seen with many H&R slug guns is that some will shoot Lightfields and nothing else. Mine in particular, I went with Lightfields first then later tried the Accu tips and Federal something with a Barnes Expander and it would not shoot them well at all.
Another good slug to try would be the Remington Copper Solid, it is an old school sabot and was one of the first I remember hearing about but it works well on deer and is usually a little cheaper than other sabot slugs.
in a 12 gauge, Lightfield Lites when zeroed at 50 yards will be the same zero for their other 12 gauge slugs, though the Hybrid and others will have a different trajectory beyond 50 yards they all have the same poi at 50 yards. They are much easier on the shoulder and wallet.
And good God get away from 3" 12 gauge slugs, no wonder you aren't getting a group.
