Put simply, the Sentry Gun is a fixed gun with sensors that aims and shoots by itself — you don't need to be standing next to it for it to do its work. Think of it as an Attack Drone without the propulsion system. You'd often find sentry guns in near-to-distant future settings, where automated combat is commonplace. They're typically weak in many scenarios in order to emphasize the need of human intervention in area security, but they do make base raiding all the more difficult for the attacking team. As such, they aren't usually capable of holding a Zerg Rush off by their lonesome, let alone a single entity.
With regards to behavior, one key difference from the Attack Drone is that it's usually completely autonomous. In more serious works, this implies a highly competent defense that you can just leave it on its own to do its work. On less serious works, the odds are its IFF won't be reliable; thanks to ridiculously poor artificial intelligence, the sentry guns are likely to shoot friendlies (and each other) by accident as much as they are likely to shoot enemies.
They take several distinct appearances. Here are the most common forms of them:
- Gun(s) mounted on legs: A gun which is put on metallic legs, usually a tripod or tetrapod. This type of Sentry Gun is commonly semi-portable and can be unmounted more easily than the rest of the subtypes.
- A surface-mounted gun: These are the guns that look like normal guns, but are firmly mounted on place and can be found on floors, ceilings and walls. Sometimes they can also hide inside the surface they are on.
- Hemisphere with barrel(s): This type of turret looks like a metallic hemisphere (sometimes more of a sphere is visible, sometimes less), usually with a groove where the gun barrel is located. These tend shoot to more futuristic stuff like Energy Balls, laser beams and more.
- Core that fires stuff: This is a very simple form and can look as simple as a square with a center which fires projectiles. These normally fire slow projectiles or laser beams.
Some real life pseudo-examples exist. While the military is reluctant to give trigger-pull authority to a computer, there are many forms of remote-operated gun platforms in development and in service today, controlled by a human operator at a distance.
- Gunslinger Girl. The Agency deploys these to defend the compound (though they are never used) and terrorists holding a nuclear power plant make use of one to fend off the Agency assault.
- The Incredibles. Edna Mode is taking Elastigirl to her secret design chamber.
Edna: (into voice-activated door control) Edna Mode.
(multi-barreled gun immediately drops out of the ceiling to cover a startled Elastigirl)
Edna: ...and guest.
- The Colonial Marines use some sentry guns to hold off a charge of alien drones in the director's cut of Aliens. The guns are arranged in two sets of two along a corridor. They watch in horror as the first two guns burn up all their ammo before they run out of targets...then one of the second pair...then...
Hudson: It aint stoppin' 'em, man... it aint stoppin' 'em... Come on baby, come on!
Hicks: D gun's dry...C gun's down to twenty...ten... [grabs his pulse rifle in preparation for a last stand]
Ripley: No, look. They're retreating. [C gun's remote display is showing ten rounds remaining, less than a second's worth of shooting]
Hicks: Next time they walk right up and knock...
Ripley: Well, they don't know that.
- The Last Starfighter. The guns protecting the Starfighter base from the Kodan meteor gun attack.
- Under Siege. A Phalanx CIWS (Close In Weapon System) aboard the battleship U.S.S. Missouri shoots down a jet fighter.
- You Only Live Twice. The "crater guns" that Blofeld ordered to be used against Tiger Tanaka's ninja army. They're quite effective in holding them off until Bond and Tanaka succeed in opening the entrance to the volcano base.
- The Empire Strikes Back. Han activates a small sentry gun, which drops out from the lower hull of the Millenium Falcon, against the snowtroopers trying to stop them escaping Hoth.
- The Australian Exploitation Film Turkey Shoot features several close-ups of twin CCTV-controlled MG3 machine guns swiveling in a menacing fashion over the inmates of the re-education camp. When the break-out finally happens they don't hit a damn thing.
- Judge Dredd. In the Aspen Penal Colony, Rico's cell has several automatic guns guarding him. That turns out to be a weakness he can exploit to escape.
- Congo. Double-barreled sensor-guided machine pistols mounted on tripods are used against the Killer Gorillas when they attack the expedition's camp.
- Cyberjack: The office building taken over by the terrorists is decked out with a laser turret defense system. Two cops investigating the scene discover this the hard way.
- The War Against the Chtorr. The protagonist turns up at a military base and presents his ID. Several cameras swing towards him "and other things that weren't cameras." Needless to say he stands very still until his ID is verified.
- In Twilight Watch, Anton is attacked by a Chinese-made mounted sentry gun. Since the sentry gun has no soul or is not attacking with ill intent, Anton is unable to detect or counter the gun with magical means, almost resulting in his death.
- This novel, more than any other, shows that human technology has reached to a point where it's a serious threat to the Others, especially if it's magically-enhanced. The book's Big Bad's plan of dealing with the most powerful Other since the Middle Ages is not some complicated spell or magical artifact. A suitcase nuke will do.
- The Indestructible Man, a Past Doctor Adventures novel by Simon Messingham. Jaime is held in an underwater prison; to get past the sentry guns he knocks out a visiting doctor and cuts out the identity chip implanted in his hand. Unfortunately the doctor wakes up and staggers after him, dying rather messily as a result.
- The Sentry Guns mentioned above on its film version also appear on the book version of Congo (and are actually a bit cooler, including silencers and the capacity of being aimed with rifle-sized laser designators). Unfortunately for the expedition, their impressive firing rate eats through their available ammunition stores at an equally impressive rate, turning them into something Too Awesome to Use for the camp's defense until the Killer Gorilla pack decides to get deadly serious.
- The Executioner. In Orbiting Omega, Mack Bolan is fired upon while infiltrating a mountain base, only to find the base is entirely automated, including rifles with drum magazines triggered by sensors. However he realises the rifles are oriented to fire over the head of anyone triggering them, as the man who set this up believes in Thou Shalt Not Kill.
- Red Planet. The rebel colonists are trapped in a building, but when a couple of them are gunned down trying to surrender, the others realise that no-one is actually watching the door except some automatic Ray Guns triggered by photosensor, so they're able to escape.
- Battlestar Galactica (1978). The guns that protected battlestars from enemy attack.
- Cleopatra 2525 had some BFGs that came out from the walls to defend the underground city if/when Baileys came in. But they hadn't been used in so long that nobody knew about them until they automatically deployed.
- Eureka: In the episode H.O.U.S.E. Rules we learn that S.A.R.A.H. has been outfitted with ion cannon sentry guns.
- One episode of MacGyver had a house defended by these.
- Star Trek: Deep Space Nine: In "Civil Defense", the replicator in the Command Center created a spherical weapon that fired phaser bolts at every non-Cardassian in the room.
- Babylon 5 has EarthForce Interceptors. Once you activate them, enemy fighters will be swatted out of the sky, and enemy fire will be shot down (unless you have the Minbari advanced weapons). They can still be overwhelmed by enough volume of fire to overheat them, though.
- In Congo, the upper-left flipper serves as a Perimeter Defense Gun, which shoots lasers (and pinballs) at an attacking gorilla.
- Shadowrun had several of these.
- The Neo-Anarchists' Guide to Real Life supplement had gun ports, which were weapons mounted in walls that hosed down with a room when activated.
- Ares Arms Sentry weapons had a modular design which allowed the addition of optional sensor packages and various weapons (e.g. machine guns and miniguns).
- Seattle Sourcebook (1990). The Renraku Arcology is protected by computer controlled sentry guns that can destroy incoming missiles.
- Early Champions products had a couple.
- The Turner Snapdown Blaster system had an autofire blaster pop out of the ceiling and attack intruders.
- The Blood and Dr. McQuark. Several automatic weapon systems were used in Dr. McQuark's base.
- Car Wars. Autoduel Quarterly Volume 1 #4 adventure "Maniac". The Elm Grove Mall had Anti Vehicular Security Stations that fired at any vehicles moving at high speed.
- Mutant Future has the Robo-Turret, which defended government installations before the end of civilization. It combined some kind of firearm with either a grenade launcher or a missile launcher.
- R. Talsorian Games' Cyberpunk supplement Night City. Automatic APEX machine guns swept the open-air lobby of the Cal Bank Building at night.
- Almost every shooter and shoot 'em up, first or third person.
- Team Fortress 2 has them as the primary kill-earner of the Engineer. It starts out as a single gun, upgrades to double chainguns, and then a further upgrade gives it rockets. Each upgrade also increases the gun's health. Subverted with the Wrangler, which allows you to aim manually, with a bonus of increased fire rate and a damage reducing shield (though even that has aim assist). In standard Team Fortress 2 logic, hit it with your wrench and your metal will repair and rearm it. It can also be replaced with the Mini-sentry by using the Gunslinger instead of the wrench, which is weaker and can't be upgraded, but cheaper and faster building, making it a good choice for offense. They are one of the most powerful weapons in the game when it comes to pure damage output, but suffer from certain weaknesses to balance it out. Obviously, they can't move (unless you pick them up, but they can't fire while packed), making them easy targets once a player is aware of them. They can't see through a spy's disguises, allowing him to easily sap and destroy unguarded sentry guns. And they have a limited range, so enemies can attack with impunity if they can stay out of range (averting this is a main advantage of the Wrangler).
- Portal has cutsey ones that follow the iPod aesthetic and say adorable things as they ventilate the player. The sequel also has these, plus defectives ones that sound like a bad stand-up comedian.
- Half-Life 2 has the tripod guns that can be moved or knocked over with the Gravity Gun.
- Half-Life has HECU sentries, mounted on a tripod. There's also the Xen Sentry Cannon.
- Tribes 2 had players on defense farm them in massive numbers.
- System Shock and its sequel used them, and let the player subvert them to his side by hacking.
- Bioshock has them, too. They're built on old rolling office chairs, and can also be hacked.
- They show up again in BioShock 2, though the player is given a "hack tool" that allows for hacking sentry guns and other electronic devices from a distance.
- In Bioshock Infinite, automata, human shaped robots with machine guns, fill the same role, and in Burial at Sea, the sentry guns from the first two games make a comeback.
- Perfect Dark had it as the secondary function of the Laptop Gun.
- Perfect Dark Zero also has surface-mounted Laptop Gun turrets. They go down easily, though.
- Borderlands has the Soldier's Scorpio Turret, that same turret as used by Crimson Lance engineers, and the massive coastal defense guns visible on some maps.
- And its sequel, Borderlands2, has Axton and his Sabre Turret, which can be upgraded to shoot rockets and slag, to stick to walls and ceilings, and to be surrounded by a force field.
- FEAR has drop-down ceiling turrets. And that deployable turret that you can chuck like a hand grenade.
- In Deus Ex, they can be hacked and turned against enemies.
- Killzone 3 's Multiplayer Engineer class
- GoldenEye (1997) had ceiling-mounted turrets, as well as guns on tripods in one level.
- Serious Sam series having cannon turrets in 1, cannon rocket and plasma turrets in 2 and minigun turrets in 3.
- Quake IV has the obligatory ceiling turrets, as well as air-dropped turrets and homing missile turrets outdoors.
- P.N.03 has several types, including standard Frickin' Laser Beams, homing missiles, and Wave Motion Gun-level One-Hit Kill death rays.
- Payday 2 has these in form of metal briefcases with a rotating foot and an integrated machine gun. An upgrade to it allows the toggling of armor-piercing rounds, at the cost of a slower rate of fire.
- Later updates introduce a SWAT Van Turret. As tall as a man, and capable of 1,000 rounds per minute, it's the bane of a lot of players.
- Even Touhou featured these at one point during the second game of the series: Submerged turrets will pop out of stage 2 and the extra stage and fire a broadside of bullets that can be tricky to dodge.
- Section 8 allows players to purchase and deploy three types of these: minigun turrets for attacking players, missile turrets for fending off vehicles, and anti-air turrets to keep players and deployables from spawningnote within their attack radius.
- The Call of Duty series staring with Modern Warfare 2 has player-deployable autoturrets.
- Evolve has these in fixed positions on all Defend maps and after hunter victories on Fusion Plant. The support character Bucket can also deploy several of his own.
- The main Marathon trilogy doesn't feature these, but many game mods do.
- RTS games usually don't count, as most stationary defenses are depicted as manned. However:
- Rise of Legends has a turret drop ability for Carlini. The parachute mounted gun drones might count, as they don't move, but they don't last very long as a defensive measure.
- Protoss turrets from StarCraft are robotic. Terran missile turrets are manned, according to the sequel at least. The automated floor and wall turrets are present in installation levels, though.
- According to the novel Shadows of the Xel'Naga, Missile Turrets are/can be automated equipment. Zerg Sunken and Spore Colonies, being living entities, sort of qualify.
- In StarCraft II, the bunker can be upgraded to have a machine gun nest on top, with threat-recognition programming derived from Zerg instincts. There is also a pop-up flamethrower turret. The Raven can deploy a stationary gun turret.
- The Turret and Rocket Turret in Dune II automatically fires at any enemy units within range. They're essential for protecting your base.
- Command & Conquer games from Tiberian Sun and beyond feature sentry guns. A rule of thumb is that sentry guns do not release soldiersnote when the gun is sold or destroyed. To further qualify the building as a sentry gun, it must be a small, low-tier defense structure with a turret. The Soviet Union (Red Alert 2 era) actually has a low-tier defense structure named the Sentry Gun (appropriately enough).
- Brink has The Engineer's pocket turret, which improves in power and durability with experience levels, going from a dinky little pea shooter to a 5mm-spewing monster.
- The Dawn of War series both lampshades and plays this straight. Most races/factions play this straight with the Listening Post structures and buildable defense turrets. Listening posts are structures used to defend requisition points and can be upgraded to automated guns of one sort or another, while the defense turrets are building/units that shoot at anything that gets close. The Orks, as ever, hang the lampshade: while their versions of the Listening Post and defense turret are guns manned by gretchen, so is every other building they get!
- Alien Swarm has the deployable IAF Advanced Sentry Gun, consisting of a heavy gauge autocannon mounted on a user-adjustable rotating stand.
- Odd example from Beyond Good & Evil. The laser turret was a one-shot kill, but it wouldn't fire on Jade unless she was spotted by a searchlight or a guard. She could sneak right by it unscathed otherwise. It basically made some sections "must-pass" stealth sequences.
- All Tower Defense games are built around this trope - you place multiple guns of different types down to prevent the enemies from reaching you.
- Bullet Bill Blasters from Super Mario Bros. series are rather well-known.
- Sentry Guns are not unheard of in Multiplayer Online Battle Arena games. For example, Heimerdinger's turrets in League of Legends and Engineer's Steam Turret (pre-change) in Heroes of Newerth act this way.
- Sentry Guns some of the more common enemies in the Contra series.
- Super Meat Boy has got rocket turrets and saw blade guns.
- Champions Online features a sentry gun power in the Inventor set. It's a summon that calls two robots that can fight and follow the player or be switched into more powerful but stationary sentry gun forms.
- "Core that shoots stuff" type in The Elder Scrolls IV: Oblivion, with floating crystals that shoot blasts of magic energy at you (and only you). Oh, and you can't do anything about them besides running and juking.
- Soul gem traps in Skyrim are the same, unless you remove the soul gem from its pedestal. Quickly solved by a well-aimed arrow or fireball.
- The SD Gundam G Generation series has turrets defending both the player and the enemy's bases, and destroying them lets the other faction take it over, which is an automatic victory. The quality of turrets gets upgraded as you progress through the game and unlock higher levels of technology.
- Fallout 3 has two types of sentry guns but they both have only three settings, 1)friendly to your enemies 2)hostile to everyone 3) off. The last one is often achieved with a shotgun, but most have an override computer near them. There are also the ubiquitous pressure plate-activated shotgun traps, and on a less lethal scale, baseball pitching machines.
- Fallout 4 has you to place sentry guns all over your settlements. Aside from machine gun turrets and shotgun turrets, you can now deploy laser turrets and missile turrets.
- Mass Effect 3 has Combat Engineers, who can deploy turrets in the middle of battle. Said turrets are, per second of exposure, among the deadliest enemies in the game.
- Certain player classes get deployable turrets of their own. The fact that they're "deployed" by throwing them like a grenade means that in the right situations, they can be used offensively instead of defensively.
- Splinter Cell has these. Since this is a Stealth-Based Game, you can't destroy them. Instead, each one has a control console near them. The goal is to sneak past the guns and either disable them or turn off their friend/foe recognition software.
- Tachyon: The Fringe has space mines that are actually armed with lasers instead of powerful explosives. They go down from one shot, though.
- Lasertowers in the X-Universe series are a cross between this and a Kill Sat. They're stationary objects deployed from a ship's cargo bay that are armed with a single powerful laser to destroy enemy ships. Unfortunately, their IFF sometimes gets glitchy and attacks neutral ships with predictable results, and they're actually fairly ineffective in their intended role in X3: Terran Conflict unless you drop a whole lot of them. (They become much more useful in Albion Prelude thanks to a buff in damage and shielding.) X3: Terran Conflict introduces Orbital Weapon Platforms, which are, in essence, enormous station-sized sentry guns capable of mounting capital ship weapons.
- Literally all of the enemies in Receiver are either flying drones or turrets on legs. Unlike most depictions, the turrets have a surprisingly small amount of ammo. But they're so accurate and fast to react that trying to trick them into wasting their ammo is a risky proposition when the amount of bullets that kills you is smaller than the number of fingers on one hand. On the other hand, they have several vulnerable points (their connecting point to the tripod, the barrel, and the sensor) that render the gun worthless with a single well-aimed shot.
- In Spider-Man (PS4), the first boss fight against the Kingpin has him activate column-mounted turrets while he watches from a safe room.
- PlanetSide 1 featured "Spitfire" turrets (among other flavors), which were created by slapping an ACE tool onto an outdoor flat surface. They were surprisingly dangerous, but could be avoided by crouch-walking so long as you avoided motion sensors. Bases had automated defense turrets mounted on the perimeter wall that would shoot upon vehicles without a stealth system, and could also be manually controlled. Automatic turrets are an upcoming feature in Planetside 2; currently all turrets are manually controlled.
- In Earth 2150, some buildings can have various turrets added that perform this role. Earth 2160 has more standard turrets. ED has wall-mounted turrets on rails that can move along the wall to reposition.
- Escape from Butcher Bay: The Double Max stage of the Butcher Bay prison is guarded by red sentry guns wherever Riddick goes. Killing foes requires getting targets in specific secluded places.
- City of Heroes had one in its Traps powerset, the Acid Mortar. Unusually, it was more of a debuffing tool than a direct attack. It did extremely low damage (around 10 base damage at maximum level, when even the weakest enemies had over 400 health), but reduced the target's damage resistance and ability to dodge by over 25%. Small wall-mounted sentry turrets are also enemies in some tech-based maps, and larger turrets protect the perimeter of Grandville, the villains' main city.
- In Champions Online, the normally-mobile Munitions Bots could be transformed into immobile but more powerful sentry turrets.
- Torbjörn from Overwatch can build a sentry turret to reinforce his team's defenses. He can upgrade his turret to level 2, and a level 2 turret is also powered up when he activates his ultimate, Molten Core.
- Damage Incorporated has podguns which look like tank turrets and fire grenades.
- Foxhole's World War-styled setting is too primitive to field actual automatic turrets, but static defenses can function as this provided that there is a Tunnel Network within range. These structures will deactivate within ten minutes if they're disconnected from the network.
- A staple of settlement defence strategy in Rimworld, however their onboard AI is noted in-universe as being rather primitive, meaning they don't care if a friendly gets in the way. (Not that your colonists are any more careful.) They also have an unfortunate tendency to explode if damaged badly enough.
- Games based on the Alien franchise, which often take their inspiration from the aforementioned Aliens, often feature the film's automated turrets as obstacles (as in Aliens: Colonial Marines) or inventory items (as in, erm.... Aliens: Colonial Marines, a total conversion mod for Doom II).
- In Dead Rising 2, take one oversized teddy bear that spots cutesy sound bites, and combine it with an LMG and you get Freedom Bear, a bandana wearing, machine gun wielding bear that occasionally spouts patriotic sound bites while firing hundreds of rounds at hordes of zombies while Chuck/Frank takes care of business. "Lets get it on!"
- The security turrets in Prey (2017) are equipped with scanning technology that allows them to detect and attack any organism with alien genetic material. This notably means that if the player starts injecting themselves with said alien genetic material in order to gain Psychic Powers, the turrets will sense the alien cells and open fire.
- Deployable turrets are one of the specialties of Arsenal ships in Ring Runner: Flight of the Sages. They come with a variety of payloads, from standard lasers and missiles to friction beams to repair rays.
- Nexus: The Jupiter Incident has space mines that are more this trope than Space Mines. For example, the mines surrounding the Shukenja Beta base can either fire missiles and railguns or lasers (both anti-ship and anti-fighter), depending on their position. They go down from a few shots, though.
- Schlock Mercenary: A common sight. They can, however, be a little quick on the trigger.
NCPD Tech: [working on a cannon] That's it. I'm pulling your twitch gate right out of the loop.
NCPD Autocannon: I said I was sorry.
- Close In Weapon Systems, abbreviated CIWS, of which there are several. Examples include but are not limited to:
- The Intelligent Munition System is halfway between a sentry gun and a land mine.
- The Samsung SGR-A1 is a South Korean sentry gun.
- H2X-40 Turret System is an automated turret armed with two AA-12 automatic shotguns.
- It's quite possible (and incredibly illegal) to obtain plans to build your own sentry gun using little more than a webcam and a remote-control solenoid on the Internet (the reason why this is illegal is because of concerns of using this as an assassination tool (which some TV shows have picked up on) and of using them for illegal poaching.
- It should be noted that the role of the sentry gun in most video games is area denial. While much ado is made over robots killing without human involvement, it's unlikely that would stop all the world's militaries if it conferred a real advantage. The more likely explanation is that land mines are a far simpler and more effective means of area denial. They're inexpensive, easy to manufacture, reliable, and long lasting. Sentry guns show up in video games land mines would remove all the challenge by making entry either impossible or a matter of pure luck. (They're also on the Geneva banned weapons list, though the US hasn't signed onto that part of the treaty, as usual).