Once you've made it out of the building via the roof, drop down and head into the building on the right to find this weapon in the safe. Input the combination 17-38-07 to gain access. Upon acquiring this weapon you'll want add Stability and Damage to it, as a headshot with this gun can take down an enemy with a single bullet.
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.