It is the barrel pin that brings the barrel into correct headspace for a weapon that has been dismantled. It is my understanding when the weapon is made, the barrel is pressed into the trunion, set for proper head space, then the pin is drilled and set. If you have one of these that the head space is off you at one time could get an oversized barrel pin to attempt to bring the barrel back into spec. Unfortunately Global Trades is out of oversized pins (of course I need one). Hope this helps. Jack