"But the good news — we got four of these robot sentries with display and scanners intact. They really kick ass, I think they'll come in handy."
―Cpl. Hicks to Ripley, regarding four sentry guns (from Aliens Special Edition)

The UA 571-C Automated Sentry Gun,[1] known colloquially as the "Robot Sentry", is a tripod-mounted automated perimeter defense system employed by the United States Colonial Marine Corps to deliver pre-set automatic fire to any hostile within range or area targets. They are equipped with M30 Autocannons and automatic thermal and movement actuated AI targeting capabilities. In 2179, the Colonial Marines on LV-426 used them successfully to keep Xenomorphs from flanking their position while waiting for rescue.


Sentry Gun in case

A UA 571-C stored in its case.

Each remote sentry unit weighs in at 19.6 kg and can be assembled in under 150 seconds.[1] The major components consist of a snap-open tripod mounting, battery pack, breech and barrel assembly, sensor unit and 500-round ammunition drum. A microwave datalink connects the sentry to a remote command console. Once emplaced, the system may be set to sweep wide arcs up to 360° (subject to terrain and emplacement restrictions), though doctrine is to set several sentries to cover smaller, interlocking fields of fire on a narrow frontage.[1] The UA 571-C mounts the pulse action M30 Autocannon[2] firing the 10×28mm High Explosive Armor Piercing round used by the Corps' M56 Smartguns. The weapon has a cyclic rate of 1,100 rpm and is air cooled, with an automatic cutout preventing the loading of any more rounds into the breech should overheating cause a chance of ammunition "cook-off".[1]


The sentry unit's sensor array is mounted above the barrel, aligned to cover a 60° cone in front of the weapon. This sensing suite consists of a cooled infrared detector in the 3-5 µm and 8-13 µm band, ambient light optics, an ultrasonic motion tracker and a lidar.[1] If a target's visual or thermal profile is known, the system may be pre-set to scan for only these specific targets using infrared or optical. However, the system is usually set to multi-spectral mode, where the sentry's software cross-correlates received data from the different sensors to obtain a full target profile.

The sensors used by the UA 571-C are able to detect Yautja even when cloaked. The information collected by the UA 571-C's sensor is automatically relayed to the visual support systems of nearby soldiers, enabling them to see cloaked Yautja as if they were uncloaked.[2]

Setup and operation[]

The UA 571-C has three operating modes — Auto-Remote, Manual Override and Semi-Automatic. When set to Auto-Remote, the sentry will interrogate all targets in its sensor cone using an Identification Friend Foe (IFF) transponder. All Colonial Marine personnel and vehicles carry an IFF transponder in their communications gear which sends back a coded radio signal when interrogated. If the IFF return is positive, the weapon will let the soldier or vehicle pass; otherwise it will open fire.[3] Enemy IFF transponders can also be detected by the UA 571-C and used to direct fire even if the target is otherwise avoiding the sentry's detection mechanisms.[1] If the system is set to Manual Override or Semi-Automatic, this information is flashed back to the command console, where the system operator can decide whether or not to fire.[3]

Before the weapon opens fire, automated servos in the tripod orient it to point at the target. The number and grouping of rounds fired in a burst is automatically selected dependent on the target profile, so that the most economical use of ammunition is made.[3]


WEY-YU Firefly

The heads-up display of a UA 571-D Ground Sentry as seen in the television series Firefly by Joss Whedon.

UA 571-D[]

While broadly similar in operation, the D variant mounts a 20 mW HF laser instead of the M30 Autocannon[2] machine gun of the regular sentry.[3]

UA 571-F[]

The F version employs a 40 mm automatic grenade launcher.[3]

Behind the Scenes[]


One of the sentry gun props.[4] The weapon's barrel shroud and sensors have been removed in this image.

Like the M56 Smartgun props, the core of the sentry guns in Aliens was a German World War II MG 42 machine gun,[5] mounted on a custom-made tripod and standing 45 inches from the ground.[6] The guns were fitted with the same custom muzzle brakes as the Smartguns and were housed in bulky fiberglass shells. The sentries are sometimes incorrectly said to be based on the earlier WWI-era Maxim LMG 08/15 machine gun due to the use of Maxim's drum magazines in the design; however, the magazine is the only part of the Maxim that was used.[5] Thorn EMI Imager Thermal Hand Helds provided by Thorn EMI[7] were placed on top of the guns to represent their sensor suites. The weapons were finished with prop battery control boxes, dressed in the same style as the main gun to look like military hardware and attached with a cable.[6] The whole ensemble was painted the same color as the Pulse Rifle props.[6]

The metal tripods on which the guns were mounted were constructed by an outside engineering firm,[5] while armorer Simon Atherton worked with the film's special effects department to install the required hydraulics that enabled the guns to rotate and elevate, simulating autonomous operation.[5] Two props were fabricated for the film, although the story implies their are four of the guns in total.[6] In terms of their visual appearance, the sentry guns are actually fairly crude compared to many of the other weapons constructed for the film. However, as the guns are never seen particularly clearly (they are only ever viewed from behind or in quick cuts) their somewhat rudimentary construction is concealed from the audience.

After filming, one of the props was recycled and painted silver for the 1996 Dennis Hopper movie Space Truckers and, today, only the prop owned by the Prop Store survives.

"Prop Store" Sentry Gun[]

Collection-item 03

The "Prop Store" Sentry Gun as it looks today.[6]

The "Prop Store" Sentry Gun is the prop owned by the Prop Store. It is the only surviving sentry gun prop from the film as the other was recycled for Space Truckers. Out of the Prop Store's guns, the Sentry Gun seems to draw the most comments from those who visit the Prop Store as most don't realize how big it is. The Prop Store Sentry Guns now sits "guarding" a corridor at the Prop Store.[6]


  • The robot sentries are among the most well-known elements cut from Aliens for its theatrical release. Scenes of the Marines setting up the weapons and their subsequent use against hordes of Warriors were later reinstated in the extended Special Edition. The guns have also become a staple of the video game franchise based on the film.
  • In Aliens: Colonial Marines, the majority of the sentry guns feature added three-digit ammunition counters. However, a single gun (one of the two weapons found outside Operations in "Mission 4: No Hope in Hadley's", as a reference to Aliens) has only a two-digit counter.
Remote Sentry MW3

The Remote Sentry in Call of Duty: Modern Warfare 3.

  • The Sentry Gun from Call of Duty: Modern Warfare 2 and the Remote Sentry from Call of Duty: Modern Warfare 3 both seem to have been inspired by the UA 571-C.



Behind the Scenes[]

See Also[]

External Links[]