Picked up a Hammer Pistol and seeing as I love playing with pistols thought I'd run with it. Unfortunately ran into an issue on the lab rat level where when attacking the chopper if I had to reload it wouldn't actually fight, but sit there invulnerable. So reloaded to an earlier point and eventually beat the chopper, however later when I picked up the Hammer Pistol again it kept reading negative ammo. Very annoying seeing as I was really looking forward to using it and the revolver.

Any knowledge on what causes this behaviour?