The compensator V2 is described as:
Reduces recoil. It is incompatible with suppressors.
You’ll find this in south-west Monte Puncu province inside Secure Barn 01. There are quite a few cartel sicarios here. The accessory case is sitting on a chest of drawers upstairs in a house in the middle of the area.
The easiest approach is definitely by night, where you can take out the snipers in the towers and sneak up into the building while the two guards are sleeping in the room with the weapon accessory. Then just knock them out and take it.
The Compensator V2 muzzle reduces recoil more than the standard compensator (in Itacua) which is great. But… this is difficult to get, and the fact that 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.
So is it worth bothering? For me, the suppressor thing’s a deal breaker, so I personally never use the compensators, but have a try and let me know in the comments how you find it, or just pick this up if you’re a completer finisher.