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Thread: Bolt Peening on lug

  1. #21
    Expert Mechanic's Avatar
    Join Date
    Aug 2013
    Lebanon, IN
    Quote Originally Posted by menzzer37 View Post
    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?
    So let me think out loud here:
    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.
    Sean K. likes this.
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  2. #22
    XCR Guru Sean K.'s Avatar
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    Nov 2008
    Agreed. Definitely keep an eye on it. Something, to me, seems amiss.
    "Necessity is the plea for every infringement of human liberty. It is the argument of tyrants; the creed of slaves."-William Pitt the Younger

  3. #23
    XCR Guru Bravo's Avatar
    Join Date
    Nov 2007
    I've had an OOB situation in a -L, and there's no heightened awareness needed - I knew it like I knew the alarmclock went off to wake me up this morning.

    On the good side, the bolt didn't appear to be damaged - on the bad side, the carrier was damaged.
    Back on the good side, Nate had everything I needed to turn that carrier group into spare parts, and have it up and going again soon.

    Probably would have run fine with the mooshed carrier, but I figure "why chance it if I don't have to?"

    All that aside, Mechanic has given you some good advice IMHO.
    Do you really think we want laws to be observed? We want them broken. There's no way to rule innocent men. The only power any government has is the power to crack down on criminals. When there aren't enough criminals, one makes them. One declares so many things to be crimes that it becomes impossible for men to live without breaking laws. Who wants law-abiding citizens? Pass the kind of laws that can’t be observed, enforced, or objectively interpreted - and you create a nation of law-breakers.

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  5. #24
    Rifleman owtlaw's Avatar
    Join Date
    Nov 2011
    Understand that if it was TiN coated, it was exposed to high heat. Very possibly high enough to draw back the heat treat. I can almost guarantee that is your problem.

  6. #25
    Newbie menzzer37's Avatar
    Join Date
    Aug 2014
    TiN is deposited at 800F. 4140 anneals 1600F. Don't think this is the issue. My shop processes 50-100 TiN bolts per month. Never had an issue with AR, MPX, MCX, AK, 556xi or any other bolt with this process.

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