Harry's Seeker reflexes came in handy as he used his right hand to keep the snapping jaws off of him while his left hand gripped Severus" derringer. He had the barrel pressed into the chin as he pulled the trigger, then pulled it again, and again. It only had two shots in it, so Harry was dry-firing it as the Inferi head was consumed with fire and he shoved it away, letting it burn and die on the ground.