This can be found in central Inca Camina province, in a rail yard controlled by a handful of patrolling cartel. The accessory case is in the office inside one of the train sheds.

I approached this via the adjacent road from the north, and took out the sicarios as I crossed the tracks to the train shed. The area is quite open, so be sure you take down enemies far enough apart that they won’t be seen, and probably use sync shots for the stealthiest approach. There is an alarm here, so it might be safest to shoot that first.

This massive magazine is for the MK-48 LMG only, which is found on the opposite side of the map in Espiritu Santo. It doubles the ammo capacity of the magazine from the standard 100 rounds to 200 rounds with a trade-off to handling. However, if you’ve not equipped a foregrip for this gun to already improve the handling, you won’t notice any trade-off at all because the handling stat is already bottomed out.