Any old handgun, usually a revolver, which is well worn-out. It refers to surplus police .38 revolvers, with a long slender barrel, which is noticeably bent. The barrel became bent after an officer used it too many time to pistol-whipping suspects into submission, by hitting them over the head with the barrel of the gun.
Don't bother aiming. You can't hit a damn thing with that bent barrel .38.