score:9
War History Online has a page about air gunner aces. To summarise, they record 17 British or Commonwealth aces, 11 Americans, and one Italian. There was at least one German ace, Erwin Hentschel, long-serving rear gunner to Hans-Ulrich Rudel.
For ground gunners, the heavy weapons are usually operated by teams, so it's hard to credit an individual on anything larger than a 20mm auto-canon. I can't find any land aces, but there was apparently a German naval ace with 12 shoot-downs.