Sentry bot

Sentry bots are a category of military robots found in Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, and Fallout 76.

Background
 The heaviest class of combat robot in the United States military, the sentry bot sacrifices speed and maneuverability for sheer resilience and overwhelming firepower. 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. 

Prototype
An antique prototype of RobCo's sentry bot line that didn't enter mass production, this prototype model features four legs on articulated mounts, with two sets of three jets mounted where the arms would be present in the production model. Hardpoints are present on arms, which are attached on the back and flank the head assembly. The weapons themselves are a pair of 5mm miniguns in a Gatling laser mount. 

Bipedal model


 An early sentry bot model, also referred to as security robot. Walking on two legs and clad in heavy armor, it carries a pair of weapons: A minigun on the left and a specially-designed multiple missile launcher on its right arm. Vision is provided by the head unit, with a single, central red eye. This old design has been largely retired by the Great War, as its height - well over two meters - and instability resulting from its center of gravity and two legs limited its performance in the field. It's a rare sight in the wasteland, with the only known facilities to use this elderly model being the Sierra Army Depot and the Enclave oil rig. 

RobCo Industries early model


 An early sentry bot model developed by RobCo Industries, designed specifically for heavy combat. The bot is divided into two halves, with the upper body containing the central processing unit, power core, and mounting the primary optical receptor. They are protected by a cast armored shell, consisting of just three pieces: The torso and upper assembly, a separate rear plate giving access to internal mechanics, and the head assembly protecting the main optical receptor. Its arms act as hardpoints, accepting a variety of weapons including miniguns, Gatling lasers, and rocket launchers. The lower body, connected by four shock absorbers and a centrally-mounted duct, contains fans venting accumulated hot air outside the chassis, preventing overheating. For movement, the lower body is fitted with three independent legs that provide it with a stable firing platform in any environment, much like a machine gun mount. The legs terminate in semi-spherical wheels (rubberized on the front, solid steel scoops on the back), giving it excellent mobility as well. Unlike the Gutsy or Assaultron, all sentry bots come with a simple synthetic personality. 

RobCo Industries late model
 The ultimate in high-end robotic engineering, the next-generation sentry bot is the heaviest and most powerful pre-War military robot design that reached operational status. It contains of two distinct assembles. The upper body, protected by a rounded, cast armored shell, contains the primary systems of the sentry bot together with the fusion core sockets. A pair of large fans help cool down the torso assembly, while hinged flaps over the fusion cores allow for emergency venting if heat buildup reaches critical levels (its only major vulnerability). The head is mounted at the front, on an articulated mount that offers a degree of mobility, and typically protected by an additional level of armor. The arms act as hardpoints, accepting a variety of loadouts including a tri-barrel minigun and a triple-shot missile launcher. An additional hardpoint is installed on the back of the sentry bot, allowing for a pair of cluster grenade launchers.

The lower body retains the original's wheeled chassis, with three legs terminating in wheels providing a stable firing platform in almost any terrain. Hydraulic actuators allow the sentry bot precise control over its position and placement, a major improvement from the previous models. However, perhaps the greatest mobility improvement in this model is the introduction of Mecanum wheels. These allow it to move in any direction in combat, without having to break line of sight to relocate. 

Gallery

 * Fallout 3 concept art


 * Fallout 4 concept art