Larry;
That would be an interesting experiment. My suspicians are that if ordinary chilled shot were loaded over card & filler wadding the 20 would show a decided advantage due to lesser deformation of shot. With premoium shot & a good qualirt shot cup I think this difference would be mosyly eliminated. What I would definitely like to see done when these tests were run would be for a half size circle (15") drawn concentric with the outer 30" one. Counting the hits in the inner circle & multiplying by 3 then dividing by the hits in the outer ring gives the ratio of central core thickening for the half dia ring.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra