The compensator is described as:
Reduces recoil. It is incompatible with suppressors.
You can pick this up in northern Malca province on a hilltop in the middle of nowhere. The accessory case is on a crate next to a helicopter, guarded by two sicarios.
You could gun these two bad guys down yourself, or take the easy option and use a sync shot. Then you’re free to just collect this muzzle accessory.
The compensator muzzle reduces recoil which is great and this is so easy to collect if you’re in the area anyway you may want to try it. The two downsides are it slightly reduces noise reduction, and when used, you can no longer equip the suppressor on your gun.
Neither of these should really be a problem for shotguns fortunately, which means this really make sense if that’s how you roll. Unless you run with the SASG-12 (found in Agua Verde) which is the only shotgun you can equip with a suppressor, you should definitely be using a compensator on your shotgun in my opinion.