There are two short stocks for ARs, and I’ll show you both locations here.
The short stock is described as:
Very short for mobility, but gives less stability while aiming.
The first is available in southern Villa Verde at the Leipez Cooperative, a small warehouse just off the road in the jungle. The accessory case is inside a shipping container outside. There are just three enemies here out front, so taking them out is very straightforward.
You can pick up the second one in southern Caimanes in Frontera, a small town. The accessory case is upstairs in a house on the south western corner of the town. There are bad guys all over the place, but if you approach from the west and avoid the roads, you only need to take down one or two directly between you and the house, while avoiding the rest completely.
These short stocks are only available for two rifles found in the same two provinces; the 556xi (found in Caimanes), and the 805 Bren A2 (which you can pick up in Villa Verde).
The short stock offer a big improvement in handling, to the detriment of accuracy. So probably equip this with caution as it’ll reduce your control of the recoil with these guns on full auto.