In researching family history I found an ancestor that was a Gunner in the Royal Field Artillery. I assumed he was working a field gun, however the thought has just occurred to me that the rank of Gunner might mean he was on a machine gun crew. Would this be likely?
Gunner is a rank comparable with a private in the infantry. https://sites.google.com/site/175brigaderfa/a-gunner-s-life
This means that he could assigned to multiple roles to perform depending on the actual posting within his brigade. Except as a driver who are mentioned as a rank of equal level but named Driver.
But from the mentioned source this sentence comes: "Gunners maintained and fired the guns," So it is most likely that he was operating an 18 pounder or 45 inch howitzer since these were the main weapons of the field artillery.
I haven't found references if the RFA also operated machine guns. I find it highly unlikely. http://www.1914-1918.net/whatartbrig.htm for more info of the different roles within a RFA brigade.