Doug! The illustration you provided is nothing more than the dimensions agreed upon to perform the gun trials carried on by the Birmingham Proof House as reported in "The Field" in March of 1891. All 39 different groups of barrels submitted for the test were turned to the same dimensions. It has nothing to do with what was considered to be safe dimensions for all sporting barrels. However, the illustration does provide a good visual of the general profile of a breechloading shotgun barrel. If these dimensions had been adhered to by British makers there would be very few of those superbly balanced lightweight game guns.
