Thanks for that data Nitrah.
The difference in Thomas' MWT recommendation is very likely related to barrel composition, and possibly service load pressure standards at the time of proof.
Turn-of-the-century pattern welded vs. decarbonized vs. pre-WWI Siemens-Martin 1030 vs. 1940s 4140 matters.

In 1925, the British 2 1/2” & 2 5/8” 12g maximum service load was reduced from 1 1/4 oz. 3 1/4 Dr.Eq. to 3 Dr. Eq. with 1 1/8 oz. shot with a mean pressure of 3 1/4 tons by LUP =
9,800 psi by Burrard’s conversion.
The 2 3/4” 12g max. service load was then 3 3/8 Dr. Eq. with 1 1/4 oz. shot with a mean pressure of 3 1/2 tons = 10,640 psi by Burrard’s conversion.
(Primarily for heavier “Waterfowl” guns)

And that is why it is difficult to provide generalized wall thickness recommendations.

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