XM123 155mm Medium Auxiliary-Propelled Howitzer.

The XM123 is an experimental weapon that fitted an auxiliary propulsion engine to a standard 155mm medium towed howitzer. The adaptation was to provide built-in mobility.

Three prototypes underwent extensive testing at Rock Island Arsenal’s Research and Development Division in 1961. The U.S. Army accepted the design in 1962. This prototype is serial number 1.