From Halopedia, the Halo wiki
The HMG-38 was considered an aging weapon as of 2495, by which time they were widely employed by the forces of the Colonial Military Administration. In this year, they were replaced in service by the VK78 Commando, and would later go on to find their way into the hands of Insurrectionists. By 2525, They were used by the guards of Colonel Robert Watts in the Eridanus cell of the Insurrection.
List of appearances