I spent almost 10 years in the Marine Corps as a Gunsmith/Armorer and a few years of that I was stationed at Parris Island Recruit Depot. We had 11,000 M16's and issued and recieved 1,000 a week. The amount of abuse the M16 rifle got in boot camp was unbelievable. These are some parts that I seen break keeping in mind this is a extreme case in a highly stressfull environment...
-Gas rings (Higher round guns), Charging handle latch, bolt lugs breaking off, extractor worn, Front sight posts bent, gas tubes clogged, ejection port cover spring, never hurts to have an extra or pivot pins, or break down pins as well as hammer and trigger pins in case they Fall out (seen it happen when the holes wear or hammer spring breaks not securing them in)...... Hope this helps
ironglowjr