I have posted this on several occasions but has been some time now. This comes from a large book entitled "The New Hunter's Encylopedia" I acquired many years ago. It is stated as being derived at, by the "Brussels Convention" of 1914;
For bbls of 12, 16 or 20 ga the minimum wall at forward end of chamber (regardless of length) for bbls made of "Ordinary Good Steel" shall be 2.3mm (.090"), for 24 & 28 ga 2.4mm (.095"). For "Special Steel" these may be reduced by .2mm (.008"). They do not give the actual taper of the bbls but indicate the "Minimum" thickness should not come till within the forward third of the bbl with it being set @ .6mm (.024").
W W Greener reported on testing some bbls which held normal loads with the "Forward" portion of the bbls so thin that after the firing he simply ripped them open with a pen knife.
Personally I would not want that .020" to come back as far as where I grip the gun, but would not be overly concerned if were out ahead.the pressure curve is beginning to level out after about 10" from breech, exact point depending upon load.
