From the space combat simulator FreeSpace 2, developed by Volition, Inc. and published by Interplay in 1999.
The GTSG Mjolnir, named after Thor's mythical hammer in Norse mythology, is the most powerful and advanced sentry gun available to the GTVA, though it is also more specialized than other sentry guns. The Mjolnir is also referred to as an RBC, for Remote Beam Cannon, because of the nature of its sole weapon.
General-purpose sentry guns, such as the Alastor, are designed to defend against fighters and bombers, and carry a few conventional energy cannons with which to do so. By contrast, the Mjolnir is designed specifically to destroy warships of cruiser size or larger (in FreeSpace, corvettes and destroyers are larger than cruisers, and cruisers are the smallest type of ship classified as a warship).
The Mjolnir carries a single heavy beam cannon. One Mjolnir is powerful enough to destroy a cruiser in under a minute (provided the cruiser doesn't destroy the Mjolnir first, of course). Mjolnirs are usually deployed in groups of three or more; this many of them are very dangerous even to a destroyer unfortunate enough to be caught in their line of fire. The Mjolnir is best used as fire support in engagements involving hostile warships.
The Achilles heel of the Mjolnir is that it is extremely vulnerable to attack by fighters and bombers, as well as warships outside its line of fire. The Mjolnir is utterly immobile, it can only fire at targets (almost) in front of it, and its weapon is only useful against warships, not small craft like fighters.
The Mjolnir is far more heavily armored than conventional sentry guns, but because it cannot defend itself against fighters and bombers, it is still only a matter of time before an undefended Mjolnir under attack by such craft is destroyed. For this reason, and because they are expensive to manufacture and deploy, Mjolnirs should be defended by fighters, warships, or other sentry guns.
The Mjolnir comes in two models. One of them has a cannon that can only fire directly in front of the sentry gun. This model has 2500 HP1. The other has 4500 HP and a weaker cannon that can be aimed to a limited degree, so it can hit targets inside a cone instead of only those that are directly in front of it.
1 For reference, the weakest cruiser in the game (GTC Fenris) has 10000 HP, and the strongest (SC Lilith) has 75000 HP. The next size up, corvettes, all have 80000 HP. The weakest sentry guns in the game (GTSG Watchdog and GVSG Ankh) have 20 HP, the above mentioned Alastor has 150 HP, and the strongest sentry gun in the game after the Mjolnir (SSG Belial) has 160 HP.
The table file that defines the types of ships in the game specifies shields with a total of 800 HP (distributed among four quadrants, as is always the case in this game). This would make the Mjolnir a lot tougher, but the model file that defines its visual shape (among a few other things) does not have a shield mesh, so it has no shields, even though the table file specifies them. This is probably an oversight by the developers.
The human readable long description (which you can read in the game, specifically in the "tech room") says the Mjolnir has two beam weapons and three missile launchers, but in reality (as defined by the above mentioned table file), it only has a single beam weapon. This must also be an oversight by the developers.
The table file says the Mjolnir is a cruiser rather than a sentry gun, even though it can't move. This is so that the AI controlling anti-warship weapons will aim the big guns at a Mjolnir, as it should. This is a bit of a hack, but it works.