It was a single shot cannon Ezekiel fired by a flexible bowden cable that was reloaded after each shot and they carried 10-12 rounds for it.
Have a look here for more info.
Ground guns of similar type had a practical rof of 10rpm. That would be a round every 6 secs, so that would tie in nicely with a reload of 3 phases in game terms.
Guynemer's first victory with a SPAD XII was a two-seater Albatros 27 July 1917 (1 shells and 8 cartridges from Vickers), next a DFW two-seater (2 shells and 30 rounds Vickers).
Seems the cockpit filled with fumes when the cannon was fired so you might want to take that into account somehow.
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