during my last tour of that dirty-dusty sand-flea infested unmentionable place. i did witness a couple of m4 bolts that failed. now even the mention of this fact will get you banned from some of the mall ninja sites. i don't know if this was some kind of QC issue with that batch of m4's or just the general abuse and bad conditions adding up to the bolts breaking the lugs off in the field.
i sort of think the cause was very long periods of sustained full automatic firing, being too dry and getting some of that flour like sand in the barrel extension--but that is just my theory.
anyway with the advent of 6.8spc, the 6.5grendel and the return of the 7.62x39 to the ar-15 type of platform. there has been a increase in bolt failures. nothing drastic--some will pretend it could blow the bolt any second. but enough for concern as the little 5.56 bolt has been streached far beyond its origional design.
the 6.5 looks to be the best candidate for a failure--with the bolt really opened up and AA's bolts have the additional "feature" of having the bolt face recessed an extra .010?? mine hasent broke yet. and i don't think it will any time soon no more than i fire it. but i wouldn't want to trust my life in the sand pit to such a thin bolt and i am thinking if you really put the rounds through it- that bolt is a likely suspect for a failure. even if it is made of kryptonite!!
7.62x39 bolts are about the same--but generally arn't cut as deep. you see on the internet more 7.62x39 bolts that have failed. some think these failed bolts came from china--but i suspect that the 7.62x39 bolts fail because 7.62x39 is still cheap compared to 6.5/6.8 and there for get a lot more use and abuse.
OK now we have settled the WHY-- lets look for a solution.
if you wanted to put a hot round in an AR type rifle-- could there be a way to strengthen the bolt without increasing its size too much? the idea is to get as much as you can without going to a bigger platform like the AR-10
i have often thought a six-lug bolt with slightly larger and thicker lugs "might" solve the problem. you would have to have a coustom "headspace" and re work the barrel extension for such an attempt.
or maybe just lugs that are a bit longer--and reach back further on the bolt maybe??
then the limit would be the barrel extension. but i have never seen a BE fail?
any thoughts on the ar bolt??