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.

Possibly not obtainable yet. (See: http://www.terrariaonline.com/threads/about-reds-vanity-set-and-s-d-m-g.108174/#post-2296339)