This depends on guns and loads. There is not much of a chance a 20 gauge with 1 5/8 oz of #9 TSS would out perform a 12 gauge with 2 1/4 oz of the same shot. I don't think the .555 is available in 12 gauge, that's a 20 gauge choke constriction. Sometimes a choke is "over choked" for the particular gun and load and blows the pattern (looks a lot like a donut with a thin pattern in the middle or splotchy). If both gauges, shooting the same shot, and both guns had their optimum choke diameter, the twelve would 99.5% of the time out shoot the twenty just based on sheer numbers of shot. Where the 20 shines is with the small TSS shot that is of such high density that it carries the lethality the same as #5 lead with twice the number of pellets on target as #5s in the same shell. If you shoot the 12, try a .660 - .665 constriction. The twenty will have half the recoil and 2-3 lbs less weight to lug around. Hope that makes some sense....
