Cast iron is such a variable.
Much of it is brittle, but much of it is not.
Add the variability of casting each individual tube - inclusions, porosity and the variabilities in cooling.
SO, if the answers to material, techniques and chance are ALL favorable, you will have a good strong tube.
Cast steel, much like cast iron in process, is much stronger.
That logically brings us to the issue of a liner - an added safety measure that overcomes the uncertainties of cast iron; hence a rule to live-by and one that has been adopted by the N-SSA.
There are pictures (somewhere in the references) there is a column of pictures that includes one of a replica 13" Seacoast (Dictator) being fired from Range 10 on Ft. McCoy some years ago.
So who makes the big ones? Who makes the BEST big ones? Not in my range of personal experience - my 4.55" is heavier than I wish, so I'm not leaning towards anything bigger.