This magazine is for 7.62x51mm chambered sniper rifles: The MK14, G28, M40A5 and SR25.
It’s located in Media Luna towards the west of the province, in a Unidad combat outpost. The accessory case is in the armoury just inside the main entrance.
There are two snipers and nine soldiers here, and I wasn’t patient enough on my approach, so take your time and pick them off and you should be able to sneak in, no problem.
This accessory extends the magazine capacity of 7.62 chambered sniper rifles such as the G28, from 10 rounds to 20 rounds. I don’t feel this is an especially necessary upgrade as I find 10 rounds are usually plenty, but if you’re a completer finisher like me, then you’ll want this anyway.