 It is a delrin bolt link pin, which links the bolt to the ram hammer.It is very popular because it is light in weight, prevents scratching of the body and, is very inexpensive compared to most other upgrades available for the ProMaster.Delrin bolts are also available, however the stock bolt has proven to be a strong point of the gun because it is also made of delrin and no evidence of performance gains has been shown with the use of an upgraded bolt.