So many things missing.......
Perhaps what you really need is one of those Zeppelin guns: standard '08 with water jacket, armoured snout and no connections on the jacket for the tanks and hoses. Filled them up on the ground and that was it; air up at 10,000 or so was cold enough that it only boiled of very lowly. There is one in the IWM main-floor exhibit hall. My grandparents both saw the Zepp being put down over London: Pup with Bucks...... Zepp burned all the way down.
Takes 500 rounds to bring the jacket to boiling, then only a pint boils of for every 500 after that. With 10 pints of the water in the thing, you can put out a lot of rounds before you run dry.
My GF was with 54 Batt ("The Kootenay Regiment"), reported making tea in the jacket of a specially-prepared Vickers (gun prepped without barrel-packing lube to keep the oil-slick off the tea). Vickers, of course, was just a lightweight Maxim upside-down. Maxim owned 50% of Vickers Sons and Maxim, which became Vickers Limited and got into aircaft, ammunition, Lugers and other fun things.
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