There are two compensated buttstocks available for assault rifles in Wildlands. The locations of both are covered here.

The compensated buttstock is described as:

Reduces recoil, but impacts pivoting mobility.

You can find the first one in Western Libertad province, at Rebel Hideout El Escondite del Cazador, an abandoned house in the middle of nowhere with four cartel soldiers around it. The accessory case is inside the house on the bed.

The enemies here are very easy to take out with a sync shot, and you’re done. You’ll then need to enter the house on the south side and kick down the door to get to the accessory case and some supplies.

The second one is east of central Inca Camina province, at Nevado Llamiru Garage, a Santa Blanca garage on a road through the mountains. A “Sicario Leader” side mission is here. The accessory case is on a shelf inside the garage.

There are seven sicarios here, including the leader in the garage. I approached at night and the sicario leader was on his own outside at the back of the garage, so I grabbed him first before killing the last of the bad guys my team mates hadn’t already taken out during my intimidating.

There are very few assault rifles that accept these compensated buttstocks. The only standard gun in the game is the Mk 17 (found in Flor de Oro). It’s strange this isn’t available for the M4A1 because there’s one for the SR25 sniper rifle in the game, built on the same platform. The other gun I know of that accepts this is the G36C which you get from the Ubisoft Club.

This is one of those accessories you’ll have to decide for yourself whether you need or not. It slightly increases accuracy and decreases mobility. I personally rely on pivoting mobility more than reduced recoil for accuracy, and I’d suggest that if you’re having an issue with recoil on an AR, consider choosing another AR.