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.
Sentry bots are the highest-level robots in Fallout 3; they are extremely durable, possessing as much health as a deathclaw but with greater Damage Resistance. Also, they do not have a weak point that can be targeted for additional damage. In fact, their heads are more durable than their torsos, though targeting the head yields a greater chance of a critical hit. Like other robots, sentry bots are equipped with a combat inhibitor, and will frenzy if the inhibitor is disabled. Exploiting this weakness can be helpful in dispatching other enemies, since sentry bots are among the most powerful foes in the game. Although they are quite fast, they have a slow turning rate, which can be used to the player's advantage.
Sentry bots are among the toughest enemies in the game, and present a serious threat to any low level player unlucky enough to encounter one. It is not uncommon to find sentry bots roaming the Capital Wasteland in groups of two or three at higher levels. These robots are mobile fortresses; their high level of health and lethality makes them the preeminent fighting machines of the Capital wastes.
Slightly darker in appearance when compared to the normal variant, the Enclave model boasts improved systems. They appear at Raven Rock and alongside Enclave squads in the wasteland. During Broken Steel they are encountered guarding Adams Air Force Base.
Military robots from before the Great War, painted in the characteristic olive green of the U.S. Army. They can be encountered at Fort Constantine, the National Guard depot and when wandering around the wasteland.
This unique sentry bot is encountered in Broken Steel at the end of the Presidential metro, near the metro to Adams Air Force Base. It holds the main fuse, which is needed to repair the nearby powerbox for the metro.
While the head of a sentry bot is the smallest part of its body, it has an increased chance for a successful hit in V.A.T.S., even when at great distances with any sort of firearm, though it can be difficult to select the head on the PC version.
Using the Robotics Expert perk on a hostile sentry bot will get the player the option to talk to it or shut it down. If the talk option is selected, the bot will only say, "Do not interfere with security operations." The player can also "talk" to a sentry bot without the perk, which will lead to the same dialog.
There is a sentry bot guarding the Citadel which will disappear after the Take it Back! quest is finished. If destroyed prior to this, however, it will respawn, allowing you a limitless supply of scrap metal, missiles, and whatever else it may carry.
Upon finishing The Superhuman Gambit, the Mechanist's robots will remain in the street (if they were destroyed previously). One or both will be sentry bots at higher levels.
There are two sentry bots in the Control Room of Raven Rock guarding the entrance to the room where President Eden can be found, and will attack the two hostile Enclave soldiers, killing them. Like other sentry bots, they can be shut down, but destroying them grants the player with only 1 experience point and negative Karma. These sentry bots also have a unique piece of dialogue, saying, "The president will see you now."
As with other types of robots, pulse grenades are especially effective against a sentry bot.
Sentry bots possess an authoritative array of speech programming, mainly expressing its current programming out loud, much like the protectron. However, sentry bots have a "deeper" voice, instilling fear in all.