Try backing off the knurled ring to move the bbl locking pin back down. The hole in the shaft of the pin inside is elongated to allow for the back and forth movement to tighten and loosen the bbl from the shotgun,,but if run too far out you hit the back end of the elongated slot in there with the assembly screw when you go to put it together.
There's also a set screw /spring tension adjustment inside the assembly for the locking pin. You get at it from the back end of the mag cap assembly. You can back down the spring tension on the pin a little if it is interferring with assembly of the screw. It will help ease pushing the screw past the spring loaded assembly if that's what's needed.
