The sentry gun is a turret seen in Jak II. It is a gun commonly used by the Krimzon Guard in the Haven sewers, support towers, and the Haven Palace. Baron Praxis placed these throughout various locations to keep Metal Heads, smugglers, and other intruders from trespassing.
Jak had to destroy four of these in the sewers for Krew during the mission "Destroy turrets in sewers" so he could continue his smuggling activities. Once Jak destroyed the turrets, a large amount of metal heads began gathering in the sewers due to there being no more imminent threat. The sheer amount can be seen most noticeably towards the end of the mission "Escort men through sewers".
The sentry gun has a simple blue-gray box-shaped turret placed on a small metal pipe which allows it to rotate 90° and cover an entire hallway. It has an elongated barrel, with a small metal cone on the front which it uses to fire. After each burst of shots fired it will release smoke from exhaust holes on the main cube and re-lock on on the enemy target. It never stops firing unless it is destroyed, which can happen with a single blow.
There is also a red version of the turret located at the support towers. These versions do not fire continuously at enemies.
An advanced version of the red one is located inside Haven Palace, it was equipped with a metal plate for defense making them indestructible. It also had a continuous automated firing mode, which would be activated when an intruder is detected, in this case it did not aim directly but simply swept from left to right.
The sentry gun will attack by firing a burst of three shots. After this it re-locks onto the target and fires again. It is easy to outmaneuver them by simply strafing from side-to-side, roll-jumping ( while moving + ) to get behind their shooting radius, or listen for the sound of it resetting, after which a double jump ( + ) can be performed to exit its target range. The Scatter Gun is a suggested weapon-of-choice for these turrets, however the Blaster may also be used at long range.