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 wheels mounted on its three legs.
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 when a low level. Like all robots, sentry bots have a combat inhibitor on the back of their chassis that, when disabled, will cause the sentry bot to go berserk and attack everything near it. Being a robot, sentry bots have no defense against electromagnetic pulses, so they can be heavily damaged by pulse grenades, mines, or any other weapon that produces EMPs. 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, possibly to avoid splash damage.
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.
An antique, prototype version that can be found in the museum inside REPCONN Headquarters. It has four wheels instead of the standard three, a set of arms stemming from its back and arching around over its shoulders. Where the arms would normally join the main body in current models, it includes 3 jets on each side, as used by Mister Gutsy/Handy robots.
Excerpt from the description of the antique model in REPCONN HQ's museum:
"Whoa, watch your step! You don't want to be facing this fearsome fellow if you accidentally stumble into a restricted area. Whether sporting the latest in dual miniguns, rockets, or laser cannons, the Sentrybot not only takes its job seriously, it also takes no prisoners! It's proof of RobCo's commitment to defense that these deadly guards are concealed in chambers throughout this facility, so let this be a warning: Watch where you step, or out'll come RobCo, guns blazing!
Despite having a melee damage 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.