This magazine is found in eastern Mojocoyo province, in a house next to a “Medication Drop” side mission to steal a helicopter. The accessory case is on the balcony of the house next to the helicopter.
This approach is very straightforward, just handle it like you would any other helicopter side mission. There are a couple of cartel bad hombres patrolling the first building you get to, then some more near the helicopter to take out afterwards. By the time you get to Mojocoyo, hopefully you’ve unlocked all your sync shots in your skill tree and can use them to great effect here.
This accessory extends the standard 20 round magazine capacity of four ARs; G2 / L85A2 / M4A1 / TAR-21 to 30 rounds. It’s a 50% increase in ammo for a small decrease in mobility for your gun stats. It’s definitely worthwhile in my book, but for even more ammo, take a look at the 50 round large magazine.