Do you have any reason to believe they would be treated any differently from any other form of soldier? POW treatment, at least in this era, is not my area of expertise unfortunately, but a work recommended to me is John Lewis-Stempel's The War Behind the Wire: The Life, Death and Glory of British Prisoners of War, 1914-18. I'll ping /u/NMW as he would know more I imagine. However nothing in my reading has indicated that machine gunners received some notably 'worse' treatment when they were captured compared to anyone else.