If one begins measuring thwe wall thickness with an accurate gage, just forward of the rim seat & continues down the bbl the following will be found in a normal gun. The OD of the bbl decreases in diameter faster than the chamber dia so the wall will decrease until the end of chamber is come to. At this point the diameter of the cone decreases faster than the OD so the wall will reverse & begiin to increase until the end of cone is arrived at. Here the bore will remain essentially constant while the OD continues decreasing, so the wall will again begin to decrease. At some point the wall will become equal to that at the juncture of the chamber & cone. The cone can thus be lengthened to this point with no loss of minimum wall thickness. Even if the cone is lengthened more than this the point where the wall becomes thinner than that of the chamber/cone juncture will be moved down the bbl where pressure is dropping so no Danger point is ordinarily established. Thus the British ruling that a gun having a lengthened cone does not need re-proofing. On the other had if the chamber itself is lengthened the OD is still decreasing faster than the chamber so an immediate reduction of wall thickness occurs at the end of the chamber.
A couple of points though worthy of note, The point of maximum pressure is reached inside the chamer itself prior to this point of minimum wall so this does not in most cases actually represent a "Weak" point. Also most American guns were not built to a carefully stuidied out minimum as are some British game guns. Most American guns can actually have the chambers lengthened without encroaching upon the minimum required wall thickness.
Of far more real concern is the overall effect of using heavier loads than the entire gun was designed around. Also do not take these comments as a recommendation to rush out & have your chambers lengthened, I do not recommend such.
I a book by Roy Dunlap "Gunsmithing" copyrited 1950 he gives minimum chamber dimensions for the 2 3/4" 12ga. He doesn't specifically say so, but I assume these were then current SAAMI specs. Minimum chamber dia was given as .798" with a minimum cone of 5° per side. This calculates to a cone length of .400" to a bore diameter of .728". But, get this, he gave a minimum length from breech to beginning of the cone as 2.6136 or slightly shorter the 2 5/8" (2.625"). Using the minimum parameters the diameter of the cone at a point 2.750" from the breeech would be .024" smaller than the chamber or .774". This chamber was sanctioned for any & all 2 3/4" sheels in guns which were otherwise built for those loads.
