Just to be certain we are all on the same page in saying "Ahead Of" the forcing cone are we speaking of the juncture of the chamber with the cone or the junction of the bore with the cone. The point where the chamber meets the cone is generally the most critical point in a shotgun bbl. Unless a very long cone the cone will normally taper more than the OD of the bbl so "Ahead of the cone (as in front of it/forward) will be thicker than behind it (breech side) as well as the pressure will be lower there than at chamber end.
A book I purchased many years ago (copyright 1966) entitled "The New Hunter's Encyclopedia" had some figures they quoted as being from the German proof houses which stated min wall at forward end of chamber should be in 12, 16 & 20 ga's 2.3mm ((.0905") & for the 24 & 28 ga's with their higher pressures 2.4mm (.0945"). This was stated to be with "Ordinary Good Steel" while if "Sppecial Steel" was used these could be reduced by no more than .2mm (.008") in the 12-20 ga's. No mention of reduction for the smaller bores. It was further stated the minimum thickness in the forward third of the bbls should be .6mm (.024").
Certainly bbls have passed proof with thinner walls in the forward portion, but personally I would not feel comfortable with any less in the chamber area & more would be better.
