Miller,
I've done quite a bit of .410 and 20 ga patterning (not so much with the 28). I think your observation concerning core thickening with the .410 is correct. Because the tall partially unprotected .410 payload (particularly 3" 11/16 or 3/4 oz) suffers so much scrubbing and setback a larger number of pellets are lost early in flight compared to a 20. Thus a 70% .410 pattern will have a hotter core than will the 70% 20 ga pattern.
Considered differently, a .410 pattern core that distributes like a 20 ga pattern will have a lower overall percentage of pellets in the 30" circle. When I ran the full choke M42 at 40 yds with RP #6 I saw a very nice 20" circle. There were maybe 10 pellets in the outer ring........yes, I said 10. Overall percentage was 67%, IIRC.
It's oversimplifying to imagine two distinct pellet populations in .410 3" shells: those with a chance to behave nicely, and those with no chance. But this conceptual works for me.
Sam
