I got an email from Ned Christiansen asking if I could find any research on bolt life for the M4, specifically anything comparing M4 to M16 bolt life. A cursory search through the research I have revealed something new and interesting to me that I thought might be of interest to the crowd here. So, Ned, I hope you don't mind if I answer your question here.
The first interesting thing is prior to about 2000, there are no published papers on bolt life at all, at least that I can find. There are one or two reports on individual failures, and their cause, but nothing on general bolt life, or bolt loading, for any of the fielded infantry weapons.
After that time there is one publicly released bolt failure study, (This one, published in 2004), that I am sure more of you are familiar with. In this report the authors state that the Army has seen an increase in bolt failures over the past few years, but unfortunately do not tell us where this data comes from. And, by where the info comes from, I mean, is it the overhaul facility procuring more bolts, is it from increased failures reported from field activities, etc? And, the other thing about this increased number of failures, is it related to increased ammunition expenditure? So, we really don't know if this is an increase in failure rate (failed bolts per rounds shot), or just an increase in number of failures. Lastly, what type of bolt failures?
Here is a good place to digress and talk about how bolts fail. There are basically two major failure mode of an M16/M4 bolt; cracking of one or more lugs, or cracking through the cam pin web. The above report goes into great detail about how and why you get lug failure, low cycle fatigue at the base of the lugs, namely the two either side of the extractor, it does not discuss web failure, which is caused by torquing of the bolt during unlocking and the snatch from the cam pin during extraction.
Back to the search for bolt life data...
In 2007, the Army published the system assessment for baseline reliability for the M4 and M16, which was an endurance test of 72,000 rounds over 12 weapons, half new and half rebuilt. To my great surprise, the mean rounds to bolt replacement for the M16 and M4 are statistically the same. Averaging around 14,000 to 15,000 rounds. Unfortunately, the cause of failure is not recorded. However, in 2015, the test was repeated with M855A1 ammunition, and all of the reported bolt failures were cracked lugs.
The results of the 2015 test indicate to me that web failure is most likely to occur in rifles with excessive carrier velocity (severely over-gassed), as none of the M4 suffered from this failure and all of the tested M4s maintained between 750 and 1000 rpm, comfortably inside the specified cyclic rate of fire of between 700 and 1025 rpm.
The last report related to bolt life I could find was dated 2017 and was a safety study to see what potential danger there was with all those cracked lugs found in 2015. In this test one to five of the seven locking lugs were removed from bolts and then fired until another lug failed or the headspace exceeded the maximum allowable limit. The data collected suggests that a bolt with up to three lugs removed (four remaining) is able to safely fire up to 720 rounds of current service ammunition before headspace begins to reach maximum allowable limits, and the bolt with just two lugs fired 6 rounds before the bolt lugs deformed to the point where it wouldn't unlock. None of them failed catastrophically. So, as a take-away, I would strongly suggest that you field strip your AR every 500 rounds or so and at least count the number of locking lugs on the bolt, and maybe clean the bolt real good and look at the lug roots with a 10X magnifier* every 5000 rounds or so.
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* They did a magnetic particle inspection to find the cracks, but looking at the pictures, most of the cracks would be visible to the naked eye, and all under magnification.
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