S.D.M.G.

The S.D.M.G (Or Space Dolphin Machine Gun) is a straight upgrade to the Megashark, as well as a considerably close competitor with the Chain Gun, with a slightly faster firing rate and same ammo usage bonus, except with a noticeably higher damage output. It also gives a +5% critical hit chance. It is currently unobtainable from the game, and the only way to get it is via an inventory editor.

Trivia

 * This item is a reference from the game Edge of Space (hence the subtext 'It came from the edge of space'), in which Terraria is also referenced.
 * S.D.M.G. is an acronym for Space Dolphin Machine Gun.
 * Space Dolphins are a race of creatures in the DC Comics universe, favored by the character Lobo.