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.
 * It may also be a reference to the book series Hitchhiker's Guide to the Galaxy. In the book series, dolphins fled the Earth when it was discovered by them that earth was scheduled to be destroyed.