You can find this magazine just south of central Koani province, in a Santa Blanca outpost. There are about half a dozen narcos here. The accessory case is in a covered shed.
I approached this from the road to the south, moving wide to the east, taking out the sniper in the tower first, then moving to the target building and letting my team mates tag enemies for me. There was just one wandering sicario to my left and two near the target building that I had to remove before I could pick this up. The rest can be ignored.
This magazine is for the SASG-12 only (found in Agua Verde), extending its standard 5 rounds, which is frankly ridiculous, to a slightly-less-than-ridiculous 10 rounds.
The SASG-12 is an automatic shotgun, meaning you can hold the trigger and it’ll pump out rounds, so you could still get through these 10 rounds quickly. Great news – there’s a larger 30 round magazine available though (in Libertad), so check that out, cos this is an awesome shotgun!