S.D.M.G.

The S.D.M.G is a straight upgrade to the Megashark, with the same fire rate and ammo usage bonus, except with a noticeably higher damage output.

Trivia

 * This item could also be a reference to the book "The Hitchhiker's Guide to the Galaxy" in which all of the planet's dolphins flew off into space to avoid the imminent destruction of Earth.
 * Currently (1.2.0.2), apart from using the Extractinator the S.D.M.G. is unavailable to obtain using legitimate means. However, an inventory editor can be used to add this weapon to a user's inventory. (See:http://www.terrariaonline.com/threads/about-reds-vanity-set-and-s-d-m-g.108174/)