You can find this magazine in central Media Luna province, opposite a gas station at a road junction (intersection). There’s a small number of Unidad soldiers around here. The accessory case is inside a Unidad bunker by the road.
There are three soldiers, two of which move around and one in the bunker. I approached at night, but because I’d driven past them on the road already they were suspicious. I did luck out though, as one of the soldiers was on the opposite side of the street and was run down by a civilian car as he crossed to come and check out my position (LOL – it’s in the video). The second soldier patrols outside around the bunker, and the third remains inside.
This magazine is specifically for the Vector .45 ACP (also in Media Luna), and increases its standard magazine capacity from a pathetic 15 rounds, to 25 rounds, which – considering the Vector rate of fire maxes out the scale – is still not really enough. Nevertheless, if you use the Vector this magazine is absolutely essential!