The long barrel is simply described as:

Increases muzzle velocity

It can be found in north-west Pucara province at an air strip on the snowy mountainside. A “Medication Air Transport” side mission is nearby to steal a plane. There are eight tangos here in total, including a captain and a heavy with an LMG. The accessory case is sitting on an A-frame bench at the side of the airstrip.

You’ll need to take out the bad guys here to get to the accessory case. The area is quite remote, and although there’s a side road leading down to the airstrip, you’ll definitely benefit from using the elevated vantage point to the west. You can then start by taking out the guy at the far end of the runway, and use sync shots to pick off the rest.

The long barrel for SMGs increases damage, range and to a lesser extent, accuracy. For this, there’s a slight handling penalty. If you use an SMG with strong handling already, such as the P90 (found in La Cruz), then equipping the long barrel may be worthwhile depending on when and how you like to use your SMG.

Bear in mind – although this doesn’t affect the gameplay at all – the long barrel makes all SMGs look a bit long and silly in my opinion, especially with a suppressor equipped as well. But consider whether you want to use accessories that help your SMG excel at shorter range, and leave this type of thing to your AR.

Either way, I’d recommend picking this up to give you that option to experiment and see how you find it for yourself. Come back and leave a comment to let me know if this works for your load out.