You can pick this magazine up in southern Flor de Oro province, at a Unidad checkpoint. A “Medican Drop” side mission is nearby to steal a helicopter. The accessory case is inside the armoury at the checkpoint, next to the road.
I approached this from the road to the north and took out the sniper in the tower first. I then had a clear run to the armoury as there were no patrolling soldiers nearby so I could make the pickup easily. As long as you’ve remained stealthy, you can then attempt the side mission or take one of the two APCs and get outta there!
This magazine is for the Mk 17 AR (which is a SCAR-H) only, which can also be found in Flor de Oro. As usual, it increases the standard 20 round magazine capacity of the gun by 50% to 30 rounds to save a bit on reloads, while sacrificing a little handling.
The Mk 17 is a great AR, so if you’re using it then you’ll almost certainly want this accessory!