From the German proof house: minimal wall thickness at end of chamber, regardless of length, for 12, 16 & 20 gauge guns should be 2.3mm (.0906") for 'ordinary good steel' or 2.1mm (.0827") if a 'Special Steel' was used. Minimal wall of .6mm (.0236") was recomended in the "forward third" of the barrel ie. what Dean said.
One of the Greeners progressively reamed the muzzles to the point that he could trim the ends with a penknife and the barrels did not rupture/split with standard loads. I wouldn't be standing next to the gun or shooter during the experiment however.
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