You'll acquire this weapon automatically at the start of The Resort portion of the game. You'll only have little ammo for this gun, so make sure to burst fire to ensure you kill the enemy without using too much ammo.
Join Ellie and Dina, as they have to overcome difficult puzzles and encounters whilst traveling through Seattle. During this adventure you'll be able to find Artefacts for interesting backstory, Trading Cards for Ellie to collect, and Weapons to make the more difficult encounters easier.
The guide for The Last of Us: Part II will include all there is to see and do including a walkthrough featuring how to take down all encounters, locate all Collectibles/Weapons, and how to solve all puzzles.