The end of the cap has been parted off the section in the tube with the cross holes.That chunk has threads for the flathead screw you see.Tighten the screw and the plug wants to pull out the end of the tube,putting the pins in shear,and taking all the play out of the joint. I wish I could take credit for the idea,but I first saw it on a homebuiltSSP 25 years ago. It does clean up the front of the gun a lot,IMO.