The Har'Peng torpedo launcher acquired as a reward from the episode "The Doomsday Device" has info in the text description that is irrelevant: "10000 damage to Doomsday Machine". There is no way at all to equip this episode reward on the temporary shuttle you are using in the episode on replay and there is no "Doomsday Device" anywhere else in the game to use it on.
Looking at the STOwiki page further down, it seems the reason for this is the Har'peng torpedo use to be a consumable that went into your ship's device slot and you had to use it on the Doomsday Device in the episode. Since it was changed to an equippable weapon, the info in the description no longer applies to the game.
Previous ship device version: The Hargh'peng Torpedo Launcher was originally a ship device that could be acquired in the mission ?The Doomsday Device? from February 2, 2010 - May 27, 2011. At the time, it was the only way to damage the Doomsday Device. The device has a full 360 degree firing arc and can be fired five times before it is consumed.