some units have a rapid fire as a separate item packed in the box when purchased. this may be installed. also, if you are using this in hand loosely,bounceback from air pressure when nail ejected will doublefire. also, you may be using the incorrect degree of nail in this unit. some generic nails, especially with wire ties, will not work in some guns. try and hold tighter against the wood and see if misfiring will stop, if not, go to a paper form nail . also, you need to be maintaining a 90 lb pressure for this gun to operate if framing nailer. low air pressure will also cause misfire.