This is my favorite self-constructed K'nex gun. it is single shot but can be very powerful depending on how many rubber bands you add to it's firing mechanism. I like this because it makes good use of flex pieces. Work the gun by pulling the pin back and its held by the yellow piece connected to the purple flex piece , pull the trigger (flex piece) to release the pin. Put bullet at the end of the gun. Bullets consist of grey connectors on blue rods. You cant make this without 2 orange flex and 1 purple flex.