The 556xi is described in the game as:
A lovechild of the AR and AK platforms.
You can find this gun west of central Caimanes province, in Los Hombres Jaguares – a cartel human hunting compound where they hold rebels in cages. The “Submarine Fleet 1” mission is nearby. The weapons case is away from the area, a little to the north-east, next to a shipping container.
If you approach this area via the road or river, you don’t actually have to worry about the enemies at the compound. They’re too far away, so you can just walk up and collect the weapon, but you’ll need to circumnavigate the wall of the compound to the east and climb over the wall to gain entry.
The 556xi scores well all-round, but scores well for an AR on range and penetration, because – despite its name – it’s chambered for 7.62 rounds, not 5.56.
If you like the AK-47 which you can pick up in Libertad, this rifle scores equal or better across the board, so you may want to give it a try. However it’s equalled on penetration and bested in range by the Mk 17 SCAR-H found in Flor de Oro, which is an excellent gun.
The 556xi shares the same magazines with the AK-12 and AK-47, so you get the usual 20 round standard magazine, and can also equip the 30 round extended mag found in Libertad, as well as the huge 50 round large drum magazine available in La Cruz, which I always recommend.