This is described in the game as:
Increases muzzle velocity
You’ll find this located in south-east San Mateo province, inside a cartel outpost. A Network Relay Station side mission is nearby. The accessory case is inside the armoury on the north side of the outpost, not far inside the main entrance.
Enemies are spaced apart and easy to pick off, so you can stroll in the main entrance to the outpost and get to the armoury easily.
The long barrel is a superb upgrade for all sniper rifles. Largely increasing range and damage, while slightly increasing accuracy and penetration, it suffers only a slight reduction in mobility.
Everyone who snipes should have this on their gun! This is one of my top accessory recommendations, and my others are listed in the description on YouTube below the video.