You can find this magazine in central Libertad province, at a small truck park off a main road. There are just four sicarios in your way. The accessory case is on a shelf outside, behind the trucks.
This accessory increases the standard 20 round magazine capacity of the G2 / L85A2 / M4A1 / TAR-21 assault rifles to a colossal 50 rounds.
Equipping this does sacrifice some mobility, however I think it’s worthwhile compared to getting caught reloading at a crucial time during a fire fight. I’d recommend you pick this up if you like to run the M4A1 or the bullpup ARs.