You’ll find this located in south-east Media Luna province, in a large Unidad observation post. The accessory case is in the armoury, not far inside the main entrance on the north-east edge.
There are three snipers and about 10 soldiers, but it’s a large base so they’re spread thin and easy to take down. You could ignore most of them as the armoury is relatively unpatrolled.
This accessory extends the magazine capacity of 9mm SMGs such as the MP5 from 20 rounds to 30 rounds, and – as usual – is highly recommended. SMGs have a high rate of fire and chew through a magazine of ammo in no time. Get this.