You can pick this magazine up in south-east La Cruz province, by a small lake with a handful of Unidad soldiers around it. The accessory case is inside the small building by the lake.
I approached this on foot along the shoreline of the lake from the north at dusk when the light is amazing! I used a sync shot to take out the soldiers grouped together by the small pier which made easy work of everybody. You have to kick down the door of the hut near the pier to enter.
This is a large 50 round magazine upgrade for three assault rifles; the AK-12 / 556xi / AK-47. It increases their magazine capacity from a standard 20 rounds to a huge 50 rounds, sacrificing some handling when equipped.
I really like these large magazines, and I don’t really notice the difference in handling – but in my opinion, having more bullets at your disposal before having to reload is definitely worth getting this accessory if you use one of these ARs.