The Super Shorty is described in the game as:

A heck of a wallop at a very short range.

It can be collected in eastern Itacua province, behind Unidad F.O.B. Amadillo. The mission “Armadillo’s Commander” is nearby. The weapons case is on the roof of a building near the river.

If you haven’t already done the story mission here, then you’ll need to take it wide around the Unidad base to approach the building. There’s usually one Unidad soldier that patrols the area in and around the building by the water. Take the stairs all the way to the roof, and the weapons crate is on the side facing the Unidad base.

This is one of the least customisable guns in the game. It comes with a built-in foregrip, but you can’t unequip it. The only addition is a long barrel (which you can get in Libertad) which slightly increases damage and range, while slightly reducing handling.

Speaking of handling, this gun scores miles better than the other two shotguns in their class, but it’s really only suited to super short ranges. Good for those missions where you need to hold an area and defend it, if the enemies are coming to you and you want to hunker down. Provided you can control the engagement and keep them at close range.

It’s quick to reload, which is just as well, because it only holds two shots and there’s no upgrade to hold any more.