I don't really know what's the dmg of the bullets right now,but as it costs 125k and is not that 'easy' to use,I mean you must be retarted to run into a guy with a pbp,rofl.Its not OP,I don't have it anymore so I can't really test,but what olirus said is true even -1 damage on a bullet would make it underpowered.
Either leave it how it is,or do random damage based on:random.