What slugs work best in a 20 guage harrington richardson ultra slug gun?

Like has been mentioned, each weapon is different. You need to get a few boxes of different manufacturers ammo and give em a try. My H&R Ultra Slugger like Lightfields, My Savage 20ga prefers Remington Accutips. Yet my Browning A-5 16ga doesn't care what I put down the barrel so I get whatever is cheapest when I buy.