This item originates from the Nintendo 3DS title Steel Diver where players pilot submarines and fire torpedoes at enemies. When the torpedoes were fired, their speed started off slow but would speed up afterwards. In Super Smash Bros. 4, the particular submarine that is used in the design of the gun is the Blue Shark, and the torpedoes it launches in SSB4 behave the same as in Steel Diver.
When used, the Steel Diver will launch torpedoes that move extremely slowly for a moment before accelerating to high speed. The torpedoes inflict decent knockback that can KO around 160%. Only two torpedoes can exist at once; they can travel slightly less than the length of Final Destination before exploding on their own. The Steel Diver holds ten shots. It is also available as an item in Smash Tour, and can be activated to hold one from the start of a battle.
In addition to the above, equipping the Dive Man or Bayonette Support Spirits will give the player a Steel Diver at a start of a match.