I was under the impression that ultima ratio regnum meant "last argument of kings."
George - That's also my understanding of the latin phrase; I mean I've seen some differences in translation like "The Final Resort/Reason of Kings", but they convey the same meaning: War had become the only option. I think the French (Louis XIV) were the first to put this on cannon, and it's the short version of "The Last Resort of Kings and Common Men", used in declarations of war.
GG, I'm missing something here; I've now reread the text of this document four or five times over, and I can't find any reference to the marking "Ultima Ratio Regum" being present on any of these
British barrels.