I had considered that aspect Waidmannsheil, but though I think it could be the issue, sort of ruled it out on the basis that the line up is metal lug on the barrel, to metal fitting in the stock fore end. The pin fits through & clicks into place.
Sometimes a hard insert, other times easy. Same with extraction.

I guess humidity must play some part, but wonder how much that (slack/tightness of pin) affects accuracy consistency.
Sorry didn't intend to hijack this thread.