the only real way this could work is if the MG dude can do everything on his own, but he has a large penalty in stamina and speed ( can only sprint so far, sprints slower ) , changes barrels slower and reloads slower
however someone can spawn an assistant, and then logically attach himself to the MGer, meaning the MGer transfers all his spare ammunition and barrels to the assistant, which makes both soldiers able to sprint at the same rate.
now when the MG is deployed, the MGer will have a sphere around him that warns the assistant he's going to far away ( but doesn't stop him from doing so ) , while the assistant in in this sphere, he will be able to utilize his weapons as per normal
when the MGer reloads or changes barrels there will be a very obvious audio and visual cue , where by the assistant faces the MGer and presses the use key to assist the gunner to speed up the reload or barrel change process.
the assistant also gets .5 points from the MGer, and a few points ( 2 maybe ) for each assist.
relatively simple, an offline training mission would clarify most of this to the noobs