At the area with the large waterfall in Peru, climb up the cliffs on the left hand side (as you face the waterfall) and continue through until you reach an area with two large waterfalls facing each other and a wooden rope bridge.
While standing on the ledge looking towards the direction that water is flowing out of the room, shoot the hook holding the bridge's rope and you will reveal a place where you can use your grappling hook.
Jump towards it and press (X) to deploy your grappling hook. Stop Swinging and press (Y) and down on the Left Stick (LS) to slide towards the bottom of the rope.
Below you and between the entrance the rope bridge was attached to and the waterfall you will see a small cave. Aim for that using the (LS). Swing the rope towards it and when you get the rope going enough press (A) to jump towards the ledge. Climb up and follow the path and you will find the shotgun and a small medpack.