First time I've seen those video clips, thanks for posting. Very interesting! And answers a question that's puzzled me.
I've noticed a small ding on each of my fired brass. Now I understand where it comes from. It's where the case contacts the sight bridge or op rod during the ejection.
Anybody know where M1 serial 1109747 is hiding? That's the one I was issued in USMC boot camp and carried until they took it away from me and gave me an M14. I looked up the serial number and noticed that it was manufactured the same month and year as my birth date. My rifle and I were almost the exact same age.