The compensator is described as:
Reduces recoil. It is incompatible with suppressors.
It can be found in north-west Malca province, next to a rebel campfire in the middle of nowhere. There’s no resistance here.
As this is a little out of the way and not knowing what to expect, I took a helicopter to the nearest road to the east and went on foot to the location. It would be simpler to just fly or drive right up to it though.
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. Although you may prefer to grab the Compensator V2 for sniper rifles available in Remanzo which reduces recoil further.
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 a sniper rifle, especially if you’ve unlocked the ADV Suppressor skill, so I personally never use the compensators, but have a try and let me know in the comments how you find it.