You can pick this magazine up in western La Cruz province, in a Unidad combat outpost located between a road and a cliff. The accessory case is inside the armoury near the main entrance to the outpost.

I approached this outpost at night time, and found you can get around the back and enter easily from the north side without being seen. If you creep about, you can rotate counter-clockwise around the inner wall of the base to the armoury and avoid most conflict. The armoury has two doors, north and south and I walked right past one of them in the dark, but there shouldn’t be any soldiers nearby.

This accessory extends the magazine capacity of five AR’s from 20 rounds to 30 rounds; the P416 / AUG A3 / 805 Bren A2 / R5 RGP / ACR. I think it’s worthwhile having as many rounds available to you as possible, but the trade off is a slight sacrifice to handling.