An elephant gun was a firearm, originally developed for use by big-game hunters for elephants and other large game in the 19th and 20th centuries. It was also used in World War I and World War II.
In 1889, the Fourth Doctor asked George Litefoot for an elephant gun and he offered a Chinese fowling gun as an alternative. Seeing that it was made in Birmingham, the Doctor accepted it and used it to kill a giant rat in the London sewers when it attacked Leela. (TV: The Talons of Weng-Chiang)
Redvers Fenn-Cooper, a Victorian hunter and member of the Royal Geographical Society, nearly shot the Seventh Doctor and Ace with an elephant gun until he was snapped out by his own reflection in Gabriel Chase during 1883. (TV: Ghost Light [+]).