Ahab

The Ahab is a mobile-exclusive Sniper added to  in the 1.9 update. It costs $17.99. It is also the only weapon that deals physical damage that also deals damage over time.

Trivia
Ahab is (as seen) in fact a crossbow based a shape of a western crossbow. This also makes Ahab currently the only "sniper rifle" to use arrows or harpoons (according to O.D. above) as ammunition & also the current Sniper based on a weapon that is not a rifle.

This weapon is likely named after Captain Ahab in the novel Moby Dick, as this weapon fires harpoons-- a common weapon of 19th century whalers.