Though I have not seen all the data available out there the largest size steel shot I have seen data for in the 20 ga is #3s (Balistics Products Inc). The largest shot I have seen in even a commercial load is #1s and there are very few of those. I suspect the reduced bore size starts to cause some problems with shot that large. I've patterned #1s and #2s in my 20 and the patterns with the #1s really start to blow apart. With the reduced pellet count and holes I suspect a goose could fly through a pattern of BBs from a 20 ga in relative safety after 30 yards. A one ounce load of BBs shows a pellet count of 72 but I have my doubts you could get that in a 3" 20 ga hull. I would suspect a 3/4 oz load (54 pellets) would be max with that pellet size as even the factory loads are showing significant dimpling on the hulls with a 1 oz load of #1s.
