Hm… what are your frame per second? It’s possible that you shoot slower if they are low.
Otherwise, I think the bullets do work even though you cannot see them (the particles may not show at low fps) but perhaps you tried to shoot when the monster was too far away : the bullets can only hit as far as what you see on the screen.
Please confirm that you did try to shoot a full clip into the enemy while it is on screen. If that fails, I’ll have a look at the code to try and figure out what goes wrong.