You are right. The earth is an oblate spheroid. I apologize.
Your analysis of the strength of a plastic shot shell is not germane. At 10 BAR it probably would blow out the primer or swell up and split. Inside a gun chamber it will act very differently. An early sign of excessive pressure is a flattened primer. Meaning that the normal primer was retaining it's shape under, let's say 800 BAR. That same primer it it weren't backed up by the breech face would blow out to hell and gone. The same analogy will hold for the case. The only way to test exactly how much hoop strength a case provides is to get two identical single barrel shotguns. Using a standard case in one increase the charge until you get a dimencial change. Now in the other gun, using similar cases but having 1/3 of their circumference removed lengthwise follow the same procedure. I believe there would a significant difference in results. Do you say the difference would only amount to 10 BAR?
