El Diablo is a scoped, high-caliber revolver and one of the strongest guns in the game, able to drop a bloater in two shots once upgraded. You find it in Tommy's Dam, on a body by the first abandoned car before the hydroelectric plant.
Its workbench upgrades include reload speed and armor piercing.
Save it for bloaters, clickers and armored humans. Against a clicker it is one of the few guns that reliably kills, and with armor piercing it punches through bandit and Firefly armor.
9mm pistol
The 9mm pistol is Joel's first gun in The Last of Us Part I, picked up from his and Tess's hideout in The Quarantine Zone before they go after Robert. It fires fast, has the most ammo around and stays useful all game as a cheap backup to your heavier guns.
Ellie's pistol
Ellie's pistol is the handgun Joel hands her in Pittsburgh, taken from a dead hunter after the plaza fight, with the warning that it is for emergencies only. You control it yourself when you play as Ellie in Lakeside Resort, and she carries it through Left Behind.
Revolver
The revolver hits harder than the 9mm but fires slower, and it is the pistol Joel draws in scripted scenes. You pick it up in The Outskirts from a dead soldier in downtown Boston, next to his field ops log.
Shorty
The Shorty is a one-handed sawed-off shotgun that sits in your pistol slot. You find it in the Pittsburgh sewers during The Suburbs, on a bench to the left just after you set off the first sound trap.