Page 1 of 2 12 LastLast
Results 1 to 20 of 30

Thread: Bolt Failure Points

  1. #1
    Join Date
    Nov 2012
    Location
    Wisconsin
    Posts
    279
    Feedback Score
    0

    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
    Join Date
    Mar 2012
    Location
    whoring myself to an NFA examiner
    Posts
    725
    Feedback Score
    8 (100%)
    http://t0.gstatic.com/images?q=tbn:ANd9GcRQ2zEv1ukzyH_aeFG6-amwPk7CUBz7b4URcTQ8lFFl-LOLSDkQ

    Now, what do you own the world?
    how do you own disorder?

  3. #3
    Join Date
    Oct 2012
    Location
    WI
    Posts
    1,154
    Feedback Score
    12 (100%)
    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.)

  4. #4
    Join Date
    Oct 2010
    Posts
    908
    Feedback Score
    16 (100%)
    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 21:53.
    A.A.S. Mechanical Engineering Technology

  5. #5
    Join Date
    Dec 2009
    Location
    Utah
    Posts
    4,998
    Feedback Score
    2 (100%)
    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
    END THE INEQUALITY OF CANDY DISTRIBUTION BASED HOW CUTE OR CLEVER A CHILD'S COSTUME IS OR WHAT NEIGHBORHOOD THEIR PARENTS CAN AFFORD TO SEND THEM TO- CANDY FOR ALL! OCCUPY HALLOWEEN!
    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  6. #6
    Join Date
    Sep 2007
    Location
    AZ
    Posts
    21,340
    Feedback Score
    8 (100%)
    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
    Join Date
    Aug 2007
    Location
    Houston, Texas
    Posts
    839
    Feedback Score
    1 (100%)
    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.

  8. #8
    Join Date
    Aug 2010
    Location
    WS, NC
    Posts
    1,156
    Feedback Score
    13 (100%)
    Take a hybrid design of lmt and kac bolts at a usgi price point and you would have a hot ticket item. Absolutely has to be drop in, kacs weakness.
    Last edited by Travis B; 02-11-13 at 08:47.

  9. #9
    Join Date
    Aug 2007
    Location
    Houston, Texas
    Posts
    839
    Feedback Score
    1 (100%)
    Quote Originally Posted by Travis B View Post
    Take a hybrid design of lmt and kac bolts at a usgi price point and you would have a hot ticket item. Absolutely has to be drop in, kacs weakness.
    That's the one thing that always bothers me about the M16 FOW, everything "has" to be drop in. It makes it incredibly hard to fix design problems if you can't change the design!
    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.

  10. #10
    Join Date
    Aug 2010
    Location
    WS, NC
    Posts
    1,156
    Feedback Score
    13 (100%)
    Quote Originally Posted by Sry0fcr View Post
    That's the one thing that always bothers me about the M16 FOW, everything "has" to be drop in. It makes it incredibly hard to fix design problems if you can't change the design!
    Well unless the OP has a quick detach barrel extension up his sleeve or can convince everyone his proprietary bolt is worth a proprietary barrel extension, the project would be worthless. But I do understand your frustration.

  11. #11
    Join Date
    Sep 2007
    Location
    AZ
    Posts
    21,340
    Feedback Score
    8 (100%)
    The bolt failure issue is less significant these days... now that we, for the most part, know to run our BCGs well lubed.

    The bolt failure issue was primarily linked to pitting.
    "You people have too much time on your hands." - scottryan

  12. #12
    Join Date
    Oct 2012
    Location
    Huntsville, AL
    Posts
    184
    Feedback Score
    0
    Two of the main factors in high cycle fatigue are peak and average stress values.

    To extend the life of a typical AR bolt, you're probably going to have to lower those two values. That entails either increasing material cross sectional area or lowering the applied force, neither of which is likely to be easy given the constraint of working within the remainder of the platform.

    You could try some more exotic things like maybe switching from shot peening to laser peening to further improve the surface quality or maybe some process to reduce the grain size of Carpenter 158 steel but I imagine the ROI on either of those is going to be small.
    Last edited by nova3930; 02-11-13 at 09:36.

  13. #13
    Join Date
    Sep 2007
    Location
    AZ
    Posts
    21,340
    Feedback Score
    8 (100%)
    I think gassing the gun correctly and not running a bunch of over powered XM193 is the way to make a bolt last for a friggin long time.
    "You people have too much time on your hands." - scottryan

  14. #14
    Join Date
    Mar 2010
    Location
    Redding Ca.
    Posts
    1,498
    Feedback Score
    0
    Look at the weak points the bolt lugs they are small and where the cam pin goes through the bolt, really thins out the metal. That is why the bolt has to be made from the C 158 steel and go through the hpt and shot peened to last. If the parts were thicker they would last longer.

  15. #15
    Join Date
    Dec 2009
    Location
    Utah
    Posts
    4,998
    Feedback Score
    2 (100%)
    Actually, high pressure testing doesn't increase bolt life. It reduces it
    END THE INEQUALITY OF CANDY DISTRIBUTION BASED HOW CUTE OR CLEVER A CHILD'S COSTUME IS OR WHAT NEIGHBORHOOD THEIR PARENTS CAN AFFORD TO SEND THEM TO- CANDY FOR ALL! OCCUPY HALLOWEEN!
    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  16. #16
    Join Date
    May 2011
    Posts
    2,796
    Feedback Score
    6 (100%)
    Quote Originally Posted by Sry0fcr View Post
    That's the one thing that always bothers me about the M16 FOW, everything "has" to be drop in. It makes it incredibly hard to fix design problems if you can't change the design!
    From a military stand point....Drop in is a must for good reason. When, not if a part breaks you cannot have half the guys using bolt design 1 with barrel extension 1 and the other half using bolt design 2 with barrel extension 2, it would make it difficult to have to keep 2 bolts in stock until all barrels/bolts are replaced.

    With a drop in bolt that fits the standard barrel extension phasing them in is easier because even though some will have older bolts they will still be comparable with each other.

    Also, generally a good bolt will shear lugs before breaking at the cam pin area.
    Last edited by sinlessorrow; 02-11-13 at 23:23.
    Quote Originally Posted by C4IGrant View Post
    Colt builds War Horses, not show ponies.
    Quote Originally Posted by Iraqgunz View Post
    This is 2012. The world is going to end this December and people are still trying to debate the merits of piece of shit, cost cutting crap AR's. Really?

  17. #17
    Join Date
    Jul 2009
    Location
    Fairbanks Alaska
    Posts
    15
    Feedback Score
    0
    If you haven't seen a broken bolt, this is what they look like.
    Attached Images Attached Images

  18. #18
    Join Date
    Nov 2012
    Location
    Wisconsin
    Posts
    279
    Feedback Score
    0
    Thanks for the replies. Lots of good info. I have a bit more research to do and then I plan to mock up my designs in an autocad program I have and hopefully find someone to machine a working physical model (not likely out of spec materials, just something 3D and functional). I'm still looking into patents and trying to figure out the best time to apply for one... not sure if I need a full spec part built, or just a model, or if I can apply for a patent simply based on a design sketch. Either way, I need to figure that out before I go public.

    Can anyone point me in the direction of a spec sheet detailing the all dimensions of a milspec M16 bolt carrier group and a standard barrel extension? Thanks.

  19. #19
    Join Date
    Nov 2012
    Location
    Wisconsin
    Posts
    279
    Feedback Score
    0
    So it looks like specs or blueprints for the bolt have not been made public... All my searching turned up was multiple people saying they are the property of Colt, released only under licensing, yada yada... and that companies that manufacture them have either reverse engineered the specs or apparently purchased a license... looks like I'll be spending some time with a pair of calipers.

    Sort of a side note, has anyone ever heard of a bolt carrier breaking? (other than the gas key coming loose)

  20. #20
    Join Date
    Feb 2013
    Location
    U.S.
    Posts
    2,307
    Feedback Score
    1 (100%)
    What is the service life of the bolt carrier?
    Every man needs a M1 Garand and 1903 Springfield rifle. Recoil is a good thing.

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •