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So let me think out loud here:They look identical. I also have a 300blk in the micro config and it looks the same as well. Wonder if it didn't get heat treated. I could do a hardness test to check. I'm guessing it's 4140? Should be between 40-50 HRC?
1. The bolt had ~1000 rounds on it before the incident/malfs. Without a history of inspections we can't be sure it wasn't gradual over time (but unlikely considering the information given indicating nominal operation up to the event). From the pics, we can be sure the damage occurred after the TiN treatment.
2. We know the damaged surface is normally only engaged during the feed portion of the cycle. From the pics, it would appear the bolt was slightly rotated when the bolt/carrier engaged. The forces in play during surface engagement should be the recoil spring and any resistance stripping a round from the magazine. These forces, theoretically, are not great enough to cause the damage depicted.
3. An out-of-battery discharge could cause those surfaces to engage, with a considerably larger force than nominal. Currently, this is only speculation as no information so far has indicated an OOB discharge occurred.
4. Bolt/Carrier geometry flaws seem unlikely due to the amount of nominal operation prior to the event.
5. Could the TiN treatment have contributed? Possibly. How many rounds were fired before and after the treatment? How many others have performed similar bolt/carrier treatments or modifications and what sort of performance have they obtained? The TiN treatment should have resulted in an increased case hardening (albeit to a relatively minor depth).
6. One suspect, misaligned ejector, seems to have been ruled out.
We know there was an event, though we simply don't have enough evidence to confirm the cause. If the damage was due to metal fatigue, probability would suggest simply replacing the bolt should result in continued nominal operation for some period. If the event was caused by something else, it is possible that it will recur with only a bolt change. We can't know when because we don't know what caused it. Maybe the chamber was hot and an OOB cook-off occurred? Maybe something else triggered an OOB discharge? Maybe the magazine or a cartridge interfered with the bolt during the feed?
One thing I do know, I would certainly heighten my awareness for a reasonable period, with frequent bolt/action inspections.