The Jamesgun is the successor of the RGM-109 Heavygun and was developed in the early UC 100s by Anaheim Electronics for the Earth Federation Forces. Although a newly designed machine, the Jamesgun's basic design follows that of the RGM-89 Jegan without compromising performance. Its simple design made it easy to use and maintain, making it a fairly popular suit. When the RGM-122 Javelin was introduced, it would share many components and designs with the Jamesgun, so many consider the two mobile suits as "brothers". The Jamesgun serves as the mainstay ground mobile suit for the Federation Forces and boasted the largest number of deployments in the Earth Sphere even during the U.C.0150s.
The RGM-119 Jamesgun was developed in the first half of the second Universal Century, and first deployed in UC 0119. However, due to delayed production, it did not play an active role during the fight with the Crossbone Vanguard in the UC 0120s. But by UC 0133, the Earth Federation was using both the Jamesgun and its successor, the RGM-122 Javelin.
The Jamesgun was used to fight the Jupiter Empire's invasion, however it was difficult to assess how the Jamesgun fared against the Jupiter Empire's mass production mobile suits. It is known that they were little match for the Death Gale Team, which destroyed multiple Earth Federation bases that would had Jamesguns defending them.
While the Jamesgun remained in service for many decades, its role was reduced to unglamorous tasks, such as providing security for material transport ships. As a result, the Jamesgun fared poorly against the more modern mobile suits employed by the Zanscare Empire during their invasion in the early UC 0150s. By U.C. 0150, production of the Jamesgun was discontinued in favor of the Javelin.