The Torchwood build-up Gun was a weapon which was kept in the boot of the Torchwood S.U.V. It came in several parts stored inside individual boxes; these parts needed to be put together before use, hence the weapon's name. It possessed great firepower, enough to destroy an entire humanoid body in one shot.
Jack Harkness shot a Nostrovite with it at Gwen Cooper's wedding. (TV: Something Borrowed)