The report is dated May 2003, so the extractor spring used was probably whatever was standard on M4s then.

Oh, and I found an old report on steel cases done back in the early 1970s. One of the curious things they noted during the endurance testing was steel cases tend to self-extract in the event of a broken extractor, where brass cases do not. So, the phenomenon may be related to case material, as well as port location relative to the end of the barrel....