Trunnions a, are cylinders cast onto either side of the gun barrel, and which connect the barrel to the carriage; they are about a ball long and just as thick. S. Cannon, Howitzer, Mould, Trunnion Hole etc.
Source: Rumpf, H. F.: Allgemeine Real-Encyclopädie der gesammten Kriegskunst (Berl. 1827)
Trunnions, cylindrical sprues on either side of the gun barrel, which serve to connect the barrel to the carriage, and whose axis, trunnion axis, must be perpendicular to the barrel axis. The intersection of both is called the bearing point. The position of the trunnions usually gives the barrel rear weight, sometimes also front weight, so that it rests on the elevation mechanism and follows its movements. Old barrels were also held in equilibrium to mitigate the deleterious effect of bucking. S. Gun, Trunnion Hole, Fighting Trunnion Hole and Traveling Trunnion Hole.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909