You can find this magazine in northern Libertad province in Pa Kollu village. This village has no cartel presence, but a “Rebel Radio” side mission is nearby. The accessory case is inside a small house in the village.
Collecting this accessory couldn’t be simpler. I just drove into the village (follow the GPS), and walked into the house. The accessory case is by the door waiting for you.
This magazine increases the standard 20 round ammo capacity of the AK-12 / 556xi / AK-47 assault rifles to 30 rounds. As a 50% capacity increase for a small loss of handling, this magazine is well worth collecting if you use one of these rifles. But they all also accept a large 50 round magazine which you might want to consider in nearby La Cruz province.