The AK-12 is described in the game as:
A sleek, modern take on the AK-47
You can find this in south-west Tabacal province, just off a road at the edge of a small airstrip. A Food Air Transport side mission is nearby where you can steal a plane. The weapons case is behind a destroyed plane fuselage in an old hangar.
There are only five sicarios around the airstrip and they wander about and spread out making them very simple to take out.
Like the ACR, this assault rifle has a 20 round magazine, with not only a 30 round extended mag accessory, but also a large 50 round drum magazine option found in La Cruz. Otherwise, this tricked out AK can accept pretty much all other AR accessories and is an ok gun.