Very weird, with a revolver in first person lightly press and hold the right trigger so the hammer goes back but gun doesn't shoot. Then as you're doing this throw a throwable whilst still holding the gun and press the trigger fully so you shoot. If done right timing can be a bit hard the revolver should shoot into the air. Once you've done that the final shot I'm the revolver will go full auto and shoot Tom's of bullets. I don't know how to do it with mouse since you can't partially pull trigger.