You can find this in eastern Caimanes province, insides Caimanes base, a large cartel base. The accessory case is inside the armoury in the middle of the base.
I used a boat to approach via the river to the north of the base. You can enter the base from that side where a fallen tree has broken the wall. Being night time, some of the enemies were sleeping, which reduced the patrols and I only had to eliminate a few sicarios before reaching the armoury easily. While you’re here, you’ll also find the SR25 sniper rifle in this base.
This large magazine increases the ammo capacity of four AR’s from 20 rounds to a huge 50 rounds; P416 / 805 Bren A2 / R5 RGP / ACR, sacrificing some handling when equipped.
I really like these large magazines, and I don’t really notice the difference in handling – but I think having more bullets available before having to reload is well worth getting this accessory if you use one of these assault rifles.