This gun is described in the game as:
A modular and reliable combat rifle
You’ll find this in central Flor De Oro province, inside M.O.B. Jaguar, the main, huge Unidad base in the region. The mission “The Black Site” is nearby. The weapons case is inside the armoury in the northern part of the base, near the entrance.
I approached from the south and had to cross into the northern part of the base via a bridge over the rail tracks. I suspect there’s a much simpler approach from the north if you can find a way in via the roads. Fortunately, because this base is immense, soldiers are thin and far between, so take out the snipers and alarms and you should be fine.
You may know this gun as the SCAR-H from other games, chambered for 7.62mm rounds, and it accepts pretty much all assault rifle accessories you can find in the game.
It shoots slow and hits hard, dealing good damage, and if I wasn’t so enamoured by the ACR I might be using this gun instead.
It has the usual 20 round standard magazine and 30 round extended mag (also found in Flor de Oro) but while there’s no large 50 round magazine available, it does have the lowest rate of fire of all the assault rifles, so you probably won’t need it.