The series 5 long-range tactical armor unit was a type of highly-advanced antimatter torpedo designed by the Druoda, a Delta Quadrant species. These torpedoes were in use as late as 2375. (VOY: "Warhead")
At just over one meter long, the torpedo was a weapon designed for mass destruction. Its explosive yield consisted of a highly focused antimatter explosion with a blast radius of two hundred kilometers. The armor unit was powered by a condensed energy matrix, which alone could power a fleet of starships. Instead of a standard computer core, there was an on-board class-11 artificial intelligence which used bio-neural circuitry to mimic humanoid synaptic functions. The intelligence was programmed to take every measure necessary to ensure that it reached its target. When separated from its explosive components and reprogrammed, it could be used to regulate a planet's weather grid for terraforming purposes.
The weapon was warp-capable, had a maximum range of eighty light years, and was protected by paratrinic shielding. Its sensors were able to detect both its position and when its systems were being tampered with. The torpedo was programmed with a targeting threshold of two light years meaning that, when it was within two light years of its target, it could not disarm or divert – not even with the correct command codes. However, the detonation sequence could be stopped by routing an electromagnetic pulse through its power matrix.
In 2375, one of these weapons was found by the USS Voyager when they answered a distress call emitted by the torpedo. The bio-neural circuitry of the weapon was malfunctioning and it believed itself to be an organic being, claiming it could not remember what its mission was. After the ship's holographic doctor beamed it to sickbay, the device suppressed The Doctor's programming and took control of his holomatrix. It then revealed its purpose and demanded that Captain Kathryn Janeway take it to its target or it would destroy Voyager.
After unsuccessful attempts to deactivate or reason with the weapon, Ensign Harry Kim discovered that the warhead was one of a small contingent that had, in fact, been ordered to stand-down by the Strategic Control Matrix. After successfully convincing the weapon's AI of the validity of its orders, Kim and the AI attempted to convince the other warheads of the mistake. However, since they had passed the two light year threshold after which nothing could cancel the attack, the AI requested that it be beamed out to join the others, reflecting that it was simply completing its mission to protect its people even if the target had changed. Voyager released the weapon and, after it and its fellows had gone to warp, it detonated itself among the remainder of the malfunctioning warheads, destroying them all. (VOY: "Warhead")