This is an often overlooked piece of dart equipment for beginners. The rubber "O Ring", also known as dart washers, prevents metal dart shafts from getting loose on the barrel of your dart. It makes the threads in the shaft good and tight. O-rings are applied on metal dart shafts by rolling the rubber ring from the top of the shaft towards the threads until it pop into place at the top of the threads. Then you tighten your shaft to the barrel and you're done. O Rings can also be very helpful with moveable dart tips. The rubber ring keeps the threads in the barrel and the dart tips tight and stops them from coming loose.