I haven't looked up anything on this subject recently, but IIRC the motive for placing the trunnions below the centerline of the bore originated in an attempt to reduce the rearward recoil of the piece, whether used on field or naval guns, and from what I remember reading, the people involved with gunnery at that time considered it a success. I'm no physicist, but even I can understand that the lower trunnions would dissipate some of the rearward energy by pivoting the breech of the gun downward into the quoin/leveling screw and into the carriage. As has already been said the major drawback of this trunnion arrangement was the increased stress and wear inflicted on the carriage. Cannon makers/artillerymen in the late 18th, and early 19th century decided it was more advantageous to go back to placing the trunnions on the centerline of the bore, perhaps because the single block field carriage was superceding the double bracket carriage, and they weren't as able to take the punishment.