I find that drawing a little hard to interpret as it shows a full length cylinder bore, no chamber, no cone & no choke. Few 1891 guns had a 3" chamber. An old Sportsman's Encyclopedia I have gave a recommended min supposedly from the German Proof house of 2.3mm (.090") at end of chamber (regardless of length of chamber) for ordinary steels. For special High strength steels it was said this could be reduced by no more than .2mm (.083"). For 28ga & smaller a slightly thicker wall was recommended. While true the smaller radius results in an increase of hoop strength, it is equally true the smaller bore are frequently loaded to higher pressures. Apparently this was considered self compensating down through about 20 ga, but as the bore went smaller pressures increased more than hoop strength, thus need a slightly thicker wall. 2.5mm (.098") & 2.3mm (.090") respectfully as I recall.
I have seen very few guns having a wall of .150" over the end of chamber. Most I have measured except for very lightweight guns seem to go from .100" to .125".
