Another option would be to go the way of the light horse artillery that lasted until the First World War and still exists in ceremonial units. The artillery piece is attached to a two wheeled carriage called, I think, a limber. Two of the gunners would ride on the limber and four more on four of the eight horses that towed the limber and gun.
One rider would stay on the team and ride them to a safe distance and the gun and limber would be kept near each other as the limber carried ammunition.
Mounting guns on wagons should be fine as long as you move the horses away from close proximity to the firing.
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