The book says that too tight of a headspace can cause the doubling ( a factor because of the rebarrel ) , but again , not just on the last rounds. Most stuff stays the same through the cycling of the eight rounds . The components of the feed / clip latch / slide hold open / area change geometry during the feeding. Look there for any bent parts or abnormal wear patterns that could be binding things up to a point where the bolt / slide movement or speed is affected . Other than that , I , too , am stumped .
Chris