This is described in the game as simply:
A powerful sniper scope
You’ll find this located in northern Koani province at the north edge of the map in a small industrial area. A Rebel Radio side mission is nearby. The accessory case is inside a shipping container.
There’s no resistance whatsoever for this accessory. The industrial area is held by rebels so you simply drive up and pick this up.
Considered the best sniper scope in the game for its high 5.5x zoom which of course lets you zoom closer to targets for improved accuracy, this is indeed excellent.
It’s a long scope and looks mean on any sniper rifle, and has clear markings. This is one of my top accessory recommendations, and my others are listed in the YouTube description below the video.