The compensator is described as:

Reduces recoil. It is incompatible with suppressors.

You’ll find this north of central Barvechos province at a cartel checkpoint at one of the entrances to Nidia Flores’ hacienda. The accessory case is upstairs in the checkpoint building.

There are only five sicarios here, who are spread around the vehicles and buildings making them easy to take out from whichever direction you approach. There’s also a jail cell with some rebels you may use as a distraction if you wish.

The compensator muzzle reduces recoil which is useful, but I wouldn’t go out of my way to get this unless you’re in the area. Unfortunately, it also slightly reduces noise reduction, and the big caveat with all compensators for all guns is that when used, you can no longer equip the suppressor on your gun.

For me, that’s a deal breaker with an SMG, so I personally never use the compensators, but have a try and let me know in the comments how you find it.