The area just in front of the chamber will be much thicker than .020. The often mentioned .020 min. thickness would be more in the middle or the muzzle end of the barrels where pressure is much reduced and the need for metal thickness is much less important.

On American guns, the are just in front of the chamber has not decreased much below .080 or .100 of the chamber area. I was given a old Belgian single shot that has less than .050 metal over the chambers and .040 on inch in front of the chamber. Thank God no idiot reamed the chambers to 3". It would have come out the side if they did. It is scary thin when you look at it and by the looks it has digested many a shell. I refuse to shoot it and keep it just to show others. No one has offered to shoot it yet.

Not directly related but a site that I find very interesting is

www.dave-cushman.net/shot/shotshellloads.html

Tons of very good information about chamber measurments and bores that has saved me from buy more than one reamed out set of barrels.