Every shotgun patterns uniquely. It’s hard to correlate chamber length to pattern. My Mossberg 835 is overbored, and throws excellent patterns with everything from 2 3/4” trap loads thru 3 1/2 magblends. When I got my 870 Special Field 20 ga I took 9 different loads to the pattern board spent 4 hours to determine it loved Federal 1 oz. 6 shot field loads. My Grouse load for 30+ years, those 4 hours have put countless birds in the game bag.