The ACR is described in the game as:
“A modern, modular assault rifle”
You can find the gun in Media Luna province, north of centre in a Unidad combat outpost at the end of La Loma village. The weapons case is inside a shipping container at the end of the outpost.
The majority of soldier activity in the outpost is near the weapons case, so it’s probably necessary to kill everybody to get to the gun. The outpost is long and thin, so it’s straightforward to work your way through it from one end to the other.
This is a great assault rifle. It’s very modular and I think accepts all accessories. Like other ARs this has a 20 round standard magazine and 30 round extended mag. But there’s also a large 50 round magazine option (found in Caimanes), which makes this AR stand above many others. This is a great gun, I can’t think of any drawbacks.