It was a beautiful morning out at Ben Avery shooting with Borderbill; who is claiming some sort of disability from a fall, but it was certainly not apparent by his shooting!
So I collected a bunch of spent wads, all from different makers, and measured the diameter of the base/cup. The depth of the cup varied from 2/16s to 3/16s and the wall of the cup would need to expand to seal a larger than gauge bore. Despite lying in the Arizona sun for an unknown period, the cups were impressively flexible.
12g - .702", .710", .719", .729" & .735"
20g - .600", .603" & .623"
28g - .550" & .588"
.410 - .405" & .408" with a much shallower cup
IMHO the cup of the 28g and 20g could expand adequately to seal a 12g .729" bore; not much chance that a .410 wad could.
