The heaviest class of combat robot in the United Statesmilitary, the sentry bot sacrifices speed and maneuverability for sheer resilience and overwhelming firepower. The sentry bot had become a standard piece of military equipment by at least 2063, with prototypes existing further back. While the assaultron is designed for melee combat and the Mister Gutsy is a jack-of-all-trades, the sentry bot's purpose is to engage the enemy at range with a variety of weapons, including miniguns, Gatling lasers, rocket launchers, and even back-mounted cluster grenade launchers, while its thick armor and multiple redundant systems allow it to survive extended engagements. It's commonly considered to be one of the most dangerous robots ever created and RobCo Industries' finest fighting robot. Late models are also provided with the ability to self-destruct to prevent enemy from recovering the robot. Early sentry bot models were issued serial numbers in the SB-#### format (model, then serial), with later models transitioning to the standardized alphanumeric registration in the ABC-###### format.
Similar to their Fallout 3counterparts, sentry bots found in the Mojave have two arms that end with deadly weapons (a minigun or Gatling laser on the right arm, and a missile launcher on the left arm), and tripod legs with wheels. They also have armored heads and, as such, receive less damage from head shots. They are programmed with an authoritative, professional military personality, in contrast to the "gung-ho," patriotic personality of the Mister Gutsy models.
Because of their high health and destructive weapons, sentry bots are formidable combatants and can be difficult to put down, especially fighting them at a low level. Like all robots, sentry bots have a combat inhibitor on the back of their chassis that, when destroyed, will cause the sentry bot to go berserk and attack everything near it. Being a robot, sentry bots have no defense against electromagnetic pulses (EMP), so they can be heavily damaged by pulse grenades, pulse mines, or any other weapon that produces an EMP. The sentry bots' use of their weapons systems depends on the distance to the target. If the target is at a long range, they'll rely on the missile launcher to damage the target while also using their secondary weapon (Gatling Laser or Minigun attachment). However, once the target reaches mid to close range, sentry bots rely primarily on their minigun/Gatling laser attachments instead of their missile launchers to avoid splash damage.
Sentry bots are armed with an arm-mounted minigun or Gatling laser on the right arm, and a missile launcher on the left arm. They have thick armor plating, and are agile in combat. Although not as common as in Fallout 3, they can still be found in various parts of the Mojave Wasteland that contain robotic guardians, such as the sacrificial chamber in Vault 11 (depending on the Courier's level) or the securitron vault.
This particular type of sentry bot can only be encountered at REPCONN headquarters, with twelve of them hidden within the building. They have the same paint scheme as standard sentry bots, but with "REPCONN" emblazoned in white across their chest. They are also much shorter in height and have glowing green eyes instead of red. If the player trips the alarm system, these bots will emerge from the hidden rooms inside the building.
A hardened sentry bot is a tougher version of the military sentry bot. They are encountered along with hardened Mister Gutsies around the crashed vertibird in the Mojave Wasteland. Hardened sentry bots have nearly twice as much health as a standard sentry bot (500 hit points as opposed to 300 hit points). However (possibly due to a developer oversight), despite what the name might suggest, they are completely unarmored with a DT of 0, unlike a standard sentry bot which has an innate DT of 18, and thus can actually survive less damage than a standard model sentry bot. One result of their increased raw health is a greater resistance to pulse grenades. This conforms with the actual electrotechnical definition of the term "hardened," meaning that hardened electronics are much more resistant to electromagnetic pulses.
It is similar to the military sentry bot in appearance, but is differently colored, with orange eyes instead of the standard red. The NCR sentry bot was originally intended to be stationed in Camp McCarran, evident in its G.E.C.K. id McCarranFieldSentryBot, but was cut from the final version of Fallout: New Vegas.
The sentry bot mark series appears only in the Fallout: New Vegas add-on Old World Blues. The multiple different variations range from being significantly weaker than regular sentry bots to being significantly tougher than the base variant.
Despite having a melee attack implemented, they do not seem to use it or have an animation for it, therefore they cannot use their melee attack. Instead they rely on their secondary weapon (the Gatling laser/minigun attachment) in close range, even in melee/unarmed ranges.