The SASG-12 is described in the game as:

A semi-auto shotgun, based on the AK-47.

You can pick this up in Agua Verde province, to the far eastern side, in a small cartel compound on the side of a hill. The weapons case is inside a shipping container.

This little compound has a watch tower and mounted machine guns, and what seems like an impenetrable gate, but this can be bypassed with explosives. It’s difficult to sneak into the compound, although there is an open gap to the left of the entrance. I started taking out enemies quietly and went loud after accidentally alerting them. But there are only eight cartel, so they’re easy to handle.

When you’re fighting in close quarters, such as hacking network relay stations, you need to use this gun. This is the best shotgun in the game by far, for two reasons:

  1. Even though it’s semi-automatic, you can hold the trigger and it’ll fire much faster than the other shotguns.
  2. There’s a huge 30 round magazine available for this gun in Libertad.

If you need more reasons, it also accepts loads of attachments. I highly recommend this gun.