The UPS pin system has a built in safety feature that can be utilized by anyone wanting to make it difficult for an upper receiver to be fitted to a lower receiver; To render a lower receiver inoperable the user can quickly remove the front pivot pin (spring & detent) and insert it through the rear takedown pin hole from the other side of the receiver; Then remove the rear takedown and allow the pivot pin to retain its detent; If the pin is rotated so that the detent registers with the groove on its end then it becomes impossible to remove the pin without removing the castle nut and end plate; It is now impossible to mount an upper receiver group to the lower and have a functioning assembly