I'm not beyond whining (begging) in the interest of science gentlemen wink and would very much like more small gauge end-of-chamber MWT numbers

The ONLY 28g barrel that I've measured was a Phil's Parker checked at Mike's cow pasture shoot in 2020. I did not keep the documentation but as I recall, both end-of-chambers measured about .105"

The earliest 28g pressure I've found were published in Frederick Toms, Sporting Guns and Gunpowder, 1897
with 3/4 oz. and + 10-14%
28 gr. Bulk (2 Dram) – 2.81 tons = 8,322 psi
18 gr. Dense (2 1/2 Dr.Eq.) – 3.01 tons = 8,994 psi

DuPont Brandywine Experimental Station data cited by Charles Askins in 1933, with DuPont Bulk Smokeless Powder
28g 2 Dram 5/8 oz. - 4.66 X 2240 = 10,438 psi + 10 - 14%

Since 28g pressures run higher than larger gauges, it would make sense that the end-of-chamber MWT would be slightly greater.

And a glorious Independence Day to each of you and your family