Gary, from your description I believe you are talking about the "joint check". If so, rotate the pin on the round part to 3 o'clock and then carefully place that part into the cut-out in the receiver, once in then lightly push the barrel forward and then close it. If you look carefully at where that slot is you will see what I mean.


If this is not the case, email me and I will try and help. jdavidw1@comcast.net