What is the name of this movie?

It has Bruce Willis and Jack Black in it the scene I remember is Willis has this mive automated Gatling Gun he was wearing sungles. Black had shoulder length hair and a beard he had a short sleeve on up shirt over a white wife beater. They were in a gry area testing the gun and Willis told Black to hold out his cigarettes he did the gun blew his are off. Then Willis told him to run he did and he shot him. Jack Black had a bit of trouble keeping his pants up in this scene.