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Terminal.png This article, M55 GUARD DOG Sentry Gun, was written by Ajax 013. Please do not edit this fiction without the writer's permission.
Production information

M55 GUARD DOG Sentry Gun

Technical specifications
  • mass: 18.9 kg
Magazine Size

700 round drum

Ammunition Type


Rate of Fire

1200 rounds/minute


1300 metres


2100 metres


Necros War




"A team, about five of them, had crept through the defence works during the night, wearing thermal cloaks to hide them from the guns. They'd gotten within about fifty metres of them, when the lead sentry opened fire, cutting the lead in two, then the other two chattered up, pinning them down. We pulled the two survivors out of that pit, covered in blood. They'd didn't even know what had went wrong. Their point man had his radio on, he didn't realise that acted as a beacon for the gun!"
―UNSC Serviceman

The M55 GUARD DOG Sentry Gun is a UNSC Sentry Gun.


The M55 GUARD DOG Sentry Gun is a remote Sentry Gun used by the UNSC. The unit weighs 18.9 kg and can be assembled by a single soldier in 140 seconds. The unit consists of a single, self contained barrel assembly, ammunition drum, sensor systems, the barrel and breech assembly, the magazine and the fire control computer and a collapsing, motorised tripod. A radio datalink connects the sentry the UNSC War Net, where in it can be interfaced by a single Marine, A.I. or routed to a remote computer, when accessed by the correct password. Once in place, the sentry gun may be programmed to sweep wide arcs up to 360 degrees, though it is subject to terrain and emplacement restrictions, though UNSC Army and Corp doctrine is to place several sentries to cover smaller, interlocking fields of fire in a narrow band in front of it. The M55 GUARD DOG Sentry Gun mounts a 7.62mm machine gun with a drum of 700 rounds. The weapon fires at 1200 rounds per minute and uses a electronically cooled, titanium-chrome cold hammer forged barrel and carbon fibre inserts.

The sentry gun's main sensor array is mounted above the barrel, aligned to cover a forward cone in front of the weapon. The sensor suite consists of a cooled infra-red detector, full colour day/night optics, a two stage passive infra-red/ultrasonic motion tracker and a LIDAR. If a target’s visual or thermal profile is recognised, the fire control system may be set just to monitor those specific targets using infra-red or optical. However, the system is usually set to multi-spectral mode, where the sentry's fire control system correlates received data from the different sensors to obtain a complete target profile. If the sentry is set to 'auto-remote', it will interrogate all targets in its sensor cone, using an Identification Friend Foe transponder. All UNSC and other AUR aligned personnel carry an IFF transponder in their communication gear which sends back a coded radio signal when interrogated. If the IFF return is positive, the weapon will let them pass, otherwise it will open fire. 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. If the returned IFF is incorrect, it can act as a beacon for the turret.

The stabilized tripod automatically adjusts the boresight along the Y-Axis. The sentry gun's fire control computer limits the number of rounds expelled based upon the target profile so that ammunition is conserved for longer operations.

The barrel is specially forged to be heat resistant, preventing thermal distortion that would affect accuracy. As a preventative measure against recoil, motion or other forms of induced shock, the barrel is set in shock-resistant, thermoplastic bedding, ensuring maximum accuracy. The gun itself has three doppler cooling banks, with two on the barrel and one on the breech. These capture heat and cool the weapon during firing, attempting to keep it as close to room temperature as possible. Alternatively, during cold weather operations, these serve to warm it up, stopping it from freezing solid. The sentry gun itself is coated in hard, damage resistant polymers, with titanium alloy plating on the 'leading face' that provides some degree of damage resistant against explosions and bullets.

UNSC Remarks

Individual and Crew Served Weaponry of the UNSCDF


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Machine Guns

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M45F Close Assault Weapon System | M108A1 Close Assault Weapon System | M77 Extreme Close Quarters Combat System

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Missile and Rocket Launchers

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Grenade Launchers

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Explosive Devices

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