Big Magnum is the final boss in Contra: Hard Corps if you take the route (Go after Deadeye Joe), then (Surrender and fight later). It is a large secret weapon powered by the Alien Cell and is piloted by Colonel Bahamut at the top of a really long elevator inside a station in space. After it is beaten the base explodes and kills all on board(including the player).
Big Magnum has several modes of attack. It fires single blasts out of its main gun as well as a few shots that hover above the player before falling straight down. It also launches a floating blast that streams beams out of it in 3 directions while turning clockwise (causing the player to run around the wall while taking shots at Big Magnum to survive).