You’ll find this magazine in south west Agua Verde province, at Carzita’s resort. The mission “The Resort” is nearby. The accessory case is on a sideboard in one of the resort buildings.
I approached this from the water in a boat at dawn, and Agua Verde looks amazing as the sun comes up. There were some sicarios patrolling the beach which are easy to take out, but watch out for the civilians! From there, you can then just go up the steps and round to the building.
This magazine is only accepted by the SR3M assault rifle – also in Agua Verde – and increases the magazine capacity 50% from the standard 20 rounds to 30 rounds. As this is the only magazine upgrade available for the SR3M and 20 rounds is usually not enough, I would definitely recommend this if you rock this AR.