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Thread: Bolt Failure Points

  1. #1
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    Bolt Failure Points

    I know this general topic has been discussed before, but I have a more specific purpose in mind. I don't want to say too much, but I will say I am working on what I hope is an improved rotating bolt design for M16/M4/AR15 patterned rifles. Although I have never experienced a bolt failure myself, and quite frankly I have not fired enough rounds through an AR15 to expect one, I know they do happen. It seems like the most common failure points people have reported are the locking lugs breaking/sheering off and cracks or complete separation at the cam pin hole. Am I correct in understanding that broken lugs are a result of improper heat treating (hardening went too deep) and cracks at the cam pin hole are simply due to a lack of material at that point? Are there any other "weak spots" on the bolt that are somewhat prone to failure? Has anyone ever had the cam pin itself break rather than the bolt?

    I'm also curious to know what kind of a market there would be for an improved bolt. I've heard everything from bolt failure being a rare occurrence to an inevitable event, but it seems like failures are more common with heavily used carbines like the M4 than mid-length or rifle length civilian AR15s. Would you replace your bolts with an improved design if one was available from a reputable manufacturer like BCM, or would you stick with the tried-and-true original design? Does anyone have a strong personal opinion one way or the other?

    Lastly, does anyone have a spec sheet for a milspec M16 bolt and carrier? I'd rather go off the actual specs when I get to the design phase rather than taking measurements off a bolt that may or may not be to exact spec.

  2. #2
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    http://t0.gstatic.com/images?q=tbn:ANd9GcRQ2zEv1ukzyH_aeFG6-amwPk7CUBz7b4URcTQ8lFFl-LOLSDkQ

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  3. #3
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    Today at the range this guy was kinda yelling at how his 300 dollar BCG just broke. I did not see what exactly happened, but he was pissed. I am sure someone would like the idea of a stronger bolt, but my BCM BCG which was half that price has kept going, but only has 3,000 maybe a couple hundred more rounds through it. . (yeah I try to count.)

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    You pretty much nailed it as far as where the failure points are but I wouldn't say failure is a result of improper heat treatment or other process as long as the failure isn't premature. AR bolts fail pretty predictably around 15k rounds. The failure is primarily due to fatigue. I ran some number for bolt stress awhile ago and if I remember correctly, the shear stress in the lugs came out to about 3,000 psi which is well below maximum shear strength for C158. However, my numbers were based on peak chamber pressure under static conditions while the bolt is locked. In addition to shear stress on the longitudinal axis, the bolt lugs are also subjected to shear stress on the lateral axis and bending stress during locking and unlocking. Although the bolt lugs do have a small radius where they meet the bolt body, that is still an area of stress concentration, as is the cam pin hole.

    As far as market for an improved bolt your two big players are LMT with their enhanced bolt design and KAC's E3 bolt, neither of which I have seen a reported failure for. The LMT bolt is expensive and KAC's bolt requires their proprietary barrel extension. I think there would be a large market for an improved bolt if it were a drop-in solution that could be made available for a price comparable to a USGI bolt.
    Last edited by Eric D.; 02-10-13 at 22:53.
    A.A.S. Mechanical Engineering Technology

  5. #5
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    An important process to AR bolt life is shot peening to relieve stress.

    Armalite shortens the height of the bolt lug opposite the extractor claiming it evens out the stress load and reduces cracking
    Ammo Before Accessories. Shoot Before Modification
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  6. #6
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    From:

    Failure Analysis of the M16 Rifle Bolt
    V.Y. Yu*, J.G. Kohl, R.A. Crapanzano, M.W. Davies, A.G. Elam, M.K. Veach
    Department of Civil and Mechanical Engineering
    United States Military Academy
    West Point, NY 10996, USA




    4. Conclusions
    The fracture of the M16 bolt resulted from a cumulative effect of high stress concentrations at the fillet radius and the additional stress concentration imposed by the presence of localized pitting at the surface. The bolt possesses many fillet regions which impose numerous areas of high stress concentration. In particular, two fillets experienced higher stress immediately adjacent to the round extractor due to the non-contiguous feature of the bolt. These two specific areas of high stress concentration also corresponded to the same location where failure of the bolt occurred in all fractured bolt specimens. Micrographs obtained from the scanning electron microscope of the fractured surface showed localized pitting at the failure initiation site. In addition, transgranular crack propagation near the pit formations in the fillet regions was observed. The localized pits formed near the locking lugs also served as high stress concentration points. The presence of pits in the material amplified the stresses of the bolt in the locking lug region which already had a high stress concentration due to the irregular geometry of the bolt. This cumulative stress concentration provides a good indicator why the crack initiated and propagated from this region.

    The wear observed in the controlled experiment indicates the mechanism of why the corrosion pits formed near the locking lug fillet by exposing the Carpenter Steel 158 base metal to the environment. Vickers microhardness readings near the fillet region show that the bolt was not uniformly case hardened. Comparison of the microhardness readings near the fillet region and 10 mm from this region show a disparity of approximately 100 units. The softer, less carburized region near the fillet contributes to the formation of a wear area after firing just 1800 rounds.
    "You people have too much time on your hands." - scottryan

  7. #7
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    markm beat me to it. KAC and Armalite have COTS solutions.
    Quote Originally Posted by Jaykayyy
    And to the guys whining about spending more on training, and relying less on the hardware, you just sound like your [sic] trying to make yourself feel superior.

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