The short barrel is described as:
Improves your mobility.
You’ll find this north-west of central Barvechos at a small cartel checkpoint on the road to Nidia Flores’ hacienda. There are only three cartel here. The accessory case is on a shelf inside the checkpoint building.
You can approach this any way you like really. With only three bad guys, I used a sync shot to take them down, then just wandered in.
The short barrel helps you manoeuvre your gun at closer quarters which sounds great for an SMG, but this accessory is not compatible with all guns. Many SMGs are already fairly manoeuvrable and I would suggest your choice of SMG is your primary concern for mobility over this accessory.