I'm not a metallurgist, but it seems to me that this would depend on both the barrel steel and on what you consider an acceptable overload margin. What you really need is for an experienced firearm designer to answer the question. I'm sure there are well established rules of thumb and that sort of thing rarely makes it into textbooks. There is a discussion and example of the math involved in a book on building a single shot rifle published by the Home Shop Machinist magazine. I don't remember the exact title offhand and my gun books are all packed away at the moment.
If I understand your question, the cartridge base is going to be inside a receiver ring as well, so the barrel diameter around the chamber going forward a bit is likely to be critical as well.
