I also had a direct ancestor in the Artillery Drivers. He - John Willis Blyth was a 1st Lieut Commissary in A Troop who mainly seemed to have safely stayed at home but he was briefly seconded to C Troop and wounded at Albuera. The difficulty in establishing where anyone in the Corps actually served is that in the field they were split up into smaller groups and came under the command of whatever Battery Commander they were seconded to. Thus they were rather an ad hoc organisation and were not (rightly or wrongly) highly regarded by the higher command of the Army. To start with the drivers were seconded to both Horse and Foot Batteries but later I believe the Horse Batteries had their own drivers, although ammunition etc could well still have been driven by the Corps. It is possible that the Death Certificate you mention was so descibed as displaying more prestige or simply that it would be better understood as the Corps of Drivers had long ceased to be by then.