The long barrel is described in the game as:

Increases muzzle velocity.

This is found in central Barvechos, south of Barvechos city at a small agricultural complex. The accessory case is just inside the door of a tiny farmhouse.

There are no enemies here, so you can drive right up and just collect the accessory with no resistance.

The long barrel increases damage and range with a penalty to handling. With LMGs, I’d recommend that if – like me – you use them exclusively for shooting down helicopters for example, and engage slower moving targets at greater distances, then this is worth equipping.

But if you run and gun with an LMG, just hosing down people, then this won’t help you at all. For that, you’d be better off with the short barrel (available in Pucara) or just leaving the barrel alone and not messing with your LMG.

Having said that, this is very easy to collect, so you may as well grab it while you’re in the area.