I am pretty sure it's an FN FNC variant which the game refers to as the fictional "AM MRS-4" probably for copyright reasons. There is no tag for the fictional name so I just tagged it as fn_fnc.
Naw, it's definitely some nameless extra who joined the Ooarai tankery team as a highly talented newbie, with the incident that caused the loss of the Ooarai school ship happening before said newbie could participate in any official match.
It does not look like an FN FNC at all. The handguard under the barrel is the most telling part as it's somewhat distinct on the FN FNC. I'm re-adding the weapon_request tag.