Sentry bots (military serial numbers SB-XXXX) are security robots made before the Great War with the express purpose of participating in the heaviest of firefights during war. Where the Mister Gutsy is a hovering platform with modest armor plating, and the protectron was originally designed for protecting business premises, the sentry bot is a powerhouse, equipped with sturdy armor plating, powerful weapons integrated into its chassis and high mobility, thanks to mecanum wheels mounted on its three legs.
The Commonwealth variants of the sentry bot are similar in overall appearance to the ones encountered in the Capital Wasteland and Mojave, but are even larger (on par in height to a human wearing power armor and nearly twice as wide), appear to have a bulkier construction and are substantially more resilient. The typical weapons loadout of the sentry bot remains the same as seen in Fallout: New Vegas, with a missile launcher and minigun being the norm (except these miniguns appear to be unique 3 barreled variants chambered for the 5.56mm round). However, alternate weaponry configurations such as dual miniguns can also be observed. Annihilator sentry bots may also possess dual back-mounted mortar launchers, further enhancing their offensive capabilities.
On top of their already formidable ballistic and explosive weaponry, sentry bots now have powerful melee attacks and a self-destruct function with equivalent power to a mini nuke. They seem more durable than previous iterations of the sentry bot in other games, however despite being extremely bulky their mobility if anything has actually improved; they are capable of moving stunningly fast and traversing extremely rough terrain (even scaling rocky outcrops with little difficulty that the Sole Survivor would struggle to jump up on). In addition; a sentry bot's entire torso is capable of rotating independently of its legs, functioning akin to a turret and allowing it to move in one direction while shooting in a different direction (potentially against targets directly behind it).
A similar variant to the Commonwealth sentry bots can be found in Fallout: New Vegas at the REPCONN headquarters museum. Newer versions in Fallout 4 appear to use Mecanum wheels that allow it to strafe right and left without having to turn.
Sentry bots are some of the rarest and the most difficult robots to defeat - fast, agile, heavily armed and resilient enough to absorb a direct hit from a mini nuke. After losing both main weapons they will initialize their self-destruct sequence and charge at the player character. Even with their ranged weaponry intact, sentry bots often charge into close combat to deliver powerful blows with their arms if one ventures too close, thus deterring attackers from any use of melee weapons. Since they explode on death with the force of a mini nuke, this tactic may prove even more dangerous to their opponent than their devastating long-range firepower. However, should a sentry bot engage the player character for too long, they will eventually enter a cooldown period, blasting steam out of their back and exposing their dual fusion cores. Hitting exposed fusion cores is the best way to cause damage to them. Players having the Penetrator perk need not wait for cooldown, and can use V.A.T.S. to target and destroy the fusion cores immediately, making this perk invaluable for fighting sentry bots. Also, unlike other robots it completely lacks a combat inhibitor, thus completely removing the possibility of using that to the player character's advantage.
Should one encounter a legendary sentry bot of any kind, keep in mind that all legendary robotic enemies explode upon death. This explosion does not override the sentry bot's built-in self-destruct, which means a legendary sentry bot will explode twice. Both explosions are equivalent to a mini nuke in power and occur with a short delay of a few seconds.
Sentry bots are a good but dangerous source of fusion cores, dropping at least two when they are destroyed, along with other valuable electronic components.
The basic version of the sentry bot, this type of sentry bot is the weakest of the sentry bots in Fallout 4 but it still a difficult combatant compared to other robots, mainly due to its high Hit Point pool, a high Perception on par with assaultrons, above average Damage Resistance and Energy Resistance and high-powered armaments (either 2 5.56mm miniguns or one and a missile launcher).
A rare, higher leveled stronger variant of the normal sentry bot. This type of sentry bot can deal lots of damage towards their enemies, have a higher Perception of 9, higher Damage Resistance and Energy Resistance, and 100 more Hit Points. The name implies that they were intended to break enemy fortifications.
A tougher and stronger variant of the sentry bot, having 150 more Hit Points (totaling 1000), higher Damage Resistance and Energy Resistance and maxed-out Perception, making it difficult to sneak up on. The annihilator is also armed with dual high-arcing mortar launchers, which at long range can hit targets even if they are behind cover. At closer range, the annihilator may switch to its standard sentry weapons.
Similar to the standard Annihilator Mk I but the mortars fire a timed explosive clump (grenades appear to be glued together) rather than a concussion shell increasing its usefulness in indoor areas and urban environments, this as well as having a further increased Hit Points and Damage Resistance and Energy Resistance results it being a formidable opponent.
An inactive combat sentry prototype MKIV can be found at the robotics disposal ground. Inside the shed near the entrance to the facility there is a terminal that has a holotape loaded. Initiating the holotape on the terminal activates the sentry bot outside, after which it immediately starts to patrol the area.
By ejecting the holotape from the terminal and using it in the player character's Pip-Boy, the Sole Survivor can command the sentry bot to go to different locations in the Commonwealth to clear of enemies, after which it will return to the junkyard. However, these locations are limited to nearby military sites. Once activated, the Sole Survivor can interact with it via the Robotics Expert perk to further the commands for it.
The space sentry (or Model X9000 according to maintenance terminal in the RobCo Battlezone) was a prototype of the sentry bot that was in development before the Great War. RobCo built several prototypes to showcase at the Nuka-Cola Corporation's Nuka-World amusement park in the Galactic Zone, primarily in the RobCo Battlezone. It features a shiny white paint scheme with gold accent colors, redesigned armor, leg mounts and wheels, a plastic dome around the head area resembling a helmet as well as new "space laser" weaponry (standard lasers renamed) and Nuka-launchers.
A higher leveled stronger variant of the normal space sentry bot, with higher Perception, Damage Resistance and Energy Resistance, and 100 more Hit Points. They appear to be the "space" equivalent of the normal sentry bot breaker.
Northwest of Murkwater construction site and south of Suffolk County charter school is what appears to be a partially submerged power station, with only the pylon towers above water. If the player character opens a door in the central pylon tower they will see a cinder block wall, with both a graffiti message that reads "YOU LOOK NICE TODAY!" and the same red skull decal used in Halloween decorations just beneath it. A sentry bot will spawn on the opposite side of the tiny building and start attacking the player character. Probably like other sentry bots, this sentry bot is level-dependent, e.g. at player character level 63 it is an Annihilator Mark II.
Directly northeast of Murkwater construction site is a memorial to fallen soldiers, guarded by an initially inactive sentry bot.
West of Somerville Place and east of Vault 95 is a sentry bot in a cage, guarding a locked storeroom on a slight hill. Picking the lock and entering the storeroom, or attacking this sentry bot directly, will activate and release it.
An inactive sentry bot can be found in the Old Robotics section of the Institute. It can be activated with the robotics terminal. It will fight the synths that are on the assembly floor, but will also be hostile to the player character.
A non-hostile sentry bot is guarding the outside of Cabot House.
With Automatron installed, the Sole Survivor can build their own sentry bots at any robot workbench. They can then be ordered to follow the Survivor like any other companion.
Owing to their sheer power, constructing sentry bots requires huge amounts of rare crafting components on par with upgrading X-01 power armor, high levels of the Armorer, Science! and Blacksmith perks as well as at least one point in Gun Nut and Robotics Expert each, assuming one wishes to access all available customization options. A fully upgraded sentry bot boasts a very large health pool, heavy armor, decent speed, the most powerful long-range weaponry of any custom-built robot type in the game (up to and including twin Fat Man mortars mounted on its back) in addition to a very strong melee attack even without specific close combat weapons attached to its arms, extraordinarily high carry weight and is capable of clearing out entire hostile strongholds on its own, given the right circumstances. On top of which; automatron sentry bots are capable of manning guard posts at settlements (with the guard tower's staircase being just wide enough to accommodate them). This grants a settlement with a decent amount of open space for the sentry bot to maneuver a level of protection far out of proportion to its supposed defensive rating and can potentially provide further assistance to any settlers during a raid if it is fitted with a support system granting damage resistance or health regeneration to allies.
As powerful as they are, the sentry bots' performance as companions suffers from a couple of drawbacks:
Their chassis is prone to overheating which often leads to temporary shutdowns in the heat of battle.
Despite their high health and heavy armor they tend to take a lot of damage very quickly, even when fighting rather weak enemies like raiders, which is largely owed to their immense size making them easy targets.
Due to the size of their torso, sometimes they are only capable of firing one of their arm weapons at a single target, reducing their effective DPS.
Although sentry bots can be outfitted with the widest array of unstable weapons, these guns are extremely fragile. The unstable Fat Man mortars usually break after just one shot whereas unstable miniguns or gatling lasers rarely get off more than two to three bursts before exploding, so mixing in some stable ranged weaponry may be a prudent choice. The sentry bot will automatically repair the weapons once the battle is over, yet it still creates several problems:
Any sentry bot without usable weapons will immediately initiate its self-destruct sequence, just like all completely disarmed robots do. An automatron that got swarmed by melee enemies and detonates in their midst is devastatingly effective, but should the robot carry ranged weaponry, it will usually try and stay close to the Sole Survivor, thus posing the hazard of killing its master in the ensuing nuclear blast unless points have been invested in the Inspirational perk.
Automatrons whose guns exploded cannot be repaired with robot repair kits during combat, which means they will instead self-destruct, take some time to reactive, then will often lose their weapons again very quickly and repeat the cycle. In comparison, robots with stable weaponry are usually much more effective.
Assuming they are armed with weaponry typically associated with other Commonwealth sentry bots (miniguns, missile launchers, cluster launchers and gatling lasers) the Automatron sentry will have a rather poor level of combat effectiveness against humanoid targets at long range (especially if they are in cover) due to the low accuracy at distant targets and or very short firing range of its weaponry. This can potentially be alleviated by using a robobrain head to increase ranged weapon's accuracy or even fitting a sniper laser weapon.
Custom sentry bots are just as large and wide as any other exemplar of their kind (assuming they are using the sentry torso and legs), so they are severely handicapped when fights move into close quarters, especially indoors. This doesn't keep them from trying to follow the Sole Survivor, but since they tend to get stuck in doors and similar obstacles, they do so by teleporting to the Survivor's side, which in turn results in them completely blocking doors and hallways extremely often.
Despite the wide array of armaments, most are either short range or have low damage and below average accuracy (such as the minigun and the missile launcher).
Contrary to most hostile sentry bots encountered in the Commonwealth, sentry bot followers seem to have serious pathfinding issues to the point where it turns them into sitting ducks on a regular basis due to them being tied to follow the player. They also perform their ramming attack less often than if they are not a follower. They have less of this problem when they are not following the player character, for example when used as provisioners or guards.
Sentry bots are especially vulnerable to the Penetrator perk, as their fusion cores can be shot at any time. This leaves an opportunity to instantly kill a fairly difficult enemy. However, if one uses any form of plasma weaponry, this effect becomes useless due to the physical behavior.
Like every other legendary version of any robotic enemy, a defeated Legendary sentry bot will always explode in an explosion comparable to a mini nuke when defeated, regardless of how damaged it was. This also happens if a non-Legendary sentry bot is set to self-destruct by hacking it.
Sentry bots that are constructed with the minigun, missile launcher or Gatling laser mods will generate a moderate amount of ammo for its primary weapon 24 in-game hours after its construction. This only occurs if the player character assigns them to a supply line.
If the sentry bot's rear wheel is crippled, it will use its two front legs to crawl and drag itself forward.
The sentry bot is unable to turn its upper body 360 degrees, which is especially noticeable when both of its legs are crippled. However it can still move its arms, thus allowing it to complete the full circle.
Despite the VDSG Catalogue saying so, sentry bots are not capable of stealth nor do they process stealth technology.
Like its previous counterparts, it suffers from very restricted head movement.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The sentry bot in Fallout 4 may be based on the T-1 tank featured in Terminator 3, as they both have dual miniguns, the ability to swivel around the waist, and are both designed for military use. However, a difference is that the T-1 propulsion system consists of tank tracks rather than mecanum wheels.
End of information based on unverified behind the scenes information.