Gun turrets are heavy weapons that appear in all the Uncharted games.
Eddy Raja's pirates own three gun turrets in Uncharted: Drake's Fortune. They are seen close to the end of Chapter 5 - The Fortress and Chapter 6 - Unlocking the Past, but can also be found at the very beginning of Chapter 11 - Trapped. Unlike turrets in other games, the Brownings have unlimited ammo. However they are weaker and have less leg protection. In reality, the turrets are Browning M1919s.
Lazarevic's Troops use gun turrets to defend certain locations. The most of them are soviet DSHK heavy machine Guns (for example the turrets in 6 and Chapter 7 - They're Coming With Us). The turrets, which are used in later chapters (for example in Chapter 19 - Siege), are GAU-19/A heavy gatling machine guns. Unlike Uncharted: Drake's Fortune, these don't have unlimited ammunition. Instead they have a certain number of ammo depending on the difficulty.
Turrets first appear in Chapter 12 - Abducted where they are setup in a set of two. The pirates use these DSHKs to keep Nate in cover while other pirates gradually move in on him or to kill him when he runs out of cover. The agents also use turrets on the top of trucks such as in Chapter 19 - The Settlement or Chapter 2 - London Underground.
Turrets appear once again in Uncharted 3: Drake's Deception, they are now a common appearance on the multiplayer maps. The turrets have 1000 bullets and cannot be destroyed. They also have shields on them so the only way to kill the user is to attack them from behind or use explosives such as grenades or RPGs. It is also possible, to shoot the gunner from the front, by aiming at the unprotected upper frontshield.
The turrets on the Chateau are on either sides of the map. Two of them are at the Team Deathmatch spawns on one side of the map and are easily accessible from the beginning. Another one is on the other side of the map.
On the Airstrip there is a turret near the Team Deathmatch spawn for the Villains mounted high. Parallel and far on the other side of the map is another turret which isn't available for use until a bomb hits the communications array above in which case it falls over and allows players to climb it and reach the turret.
On London Underground there is a turret right outside of the subway entrance, where the Villains first get off the train.