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Thread: Broken Bolts?

  1. #21
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    Not trying to start a debate, just reviewed Hogdon's load data and their published max load is 29.8gr XBR @ 49,300 PSI-24" tube. Sierra is much lower for the 107 SMK @ 28.5. Thoughts? Different Projectile for Hogdon?



    That got me thinking maybe my barrel extension isn't square after all and the bolt broke from not hitting square. Thank you amazon USB barrel Cam:



    Looks like i was shooting for a while with a missing tooth, anything else look out of place? Lugs next to the sheared one seem to have more wear than the others???

    Last edited by brut28481; 12-04-2017 at 01:00 AM.

  2. #22
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    Brut:

    You are still over the max recommended load. Even by .2 grains providing your scale was accurate.

    I won't belabor the point. Max out loads for a Grendel, particularly with a fast powder for that specific bullet, and you will shear lugs way before they should shear. Go over max and you will shear them sooner.

    LR55

  3. #23
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    Agreed! Do you think I might also have a bolt face tolerance issue that also contributed to early lug failure? If so, how do I check and prevent with new new bolt in addition to dropping my load? Thanks!

  4. #24
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    Quote Originally Posted by brut28481 View Post
    Agreed! Do you think I might also have a bolt face tolerance issue that also contributed to early lug failure? If so, how do I check and prevent with new new bolt in addition to dropping my load? Thanks!
    You previously said "the receiver was lapped before I installed barrel so should be square". Does that mean you or someone else lapped it, and was squareness actually verified? If you did not personally confirm the upper and barrel are true, then I would at least disassemble the upper and recheck before proceeding.

    Once you have a trued upper and extension, scrub the extension until it is squeaky clean. I would then apply dykem blue to the lug faces of your new, unlubricated bolt. Chamber an empty case and close the bolt. Repeat this a few times and check the lugs for even contact.

    Finally, once you've confirmed even contact, discard your previous load data and work up something which does not exceed maximum loads.

    I always try to equate hot-rodding an AR to hot-rodding cars. You can squeeze more horsepower out of a given engine, but in doing so, you will either shorten its useful life or suffer a KB in the form of a dropped rod, blown head or busted tranny or any combination of the above.

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  5. #25
    Chieftain NugginFutz's Avatar
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    One more thing, from an older post by Bill Alexander on the subject of bolts and hand loading...

    The issue of bolts is complex. Steel type is the beginning and end of a saga but the interaction between a bolt and a barrel extension can negate all the good work elsewhere. I routinely inspect barrel extensions and it is not unusual to see the pitch diameter be off axis to both the centre line of the part but also with marked angularity to the lug surface. Sure, the barrel shoulder should square everything up but if you pull a class 3 thread on a 60 HRC part up to a soft shoulder on the barrel the final result is more luck than judgement. Seen more than a few barrel makers fix the problem with loose threads, lots of problems. Have fun with that build!

    The Grendel is exactly what it says on the box. We shoehorned a bigger cartridge into the gun using the tolerances and clearances. As such you have to respect that the parts now have to be right on the money both dimensionally and also mechanically. Something drifts and you will get problems.

    ...

    And finally hand loading. LR is correct that the the Grendel must be approached with caution. If you simply load it up and then look for flat primers you have already killed your parts. The MAOP is a very mild 50,000 psi, proof is 67,340 psi. I cannot see the difference on the case or the primers when firing the two. That said it is quite capable of meeting the performances we built in while remaining durable. I have test mules that have run over 8000 rounds and are still ticking along. But a small case will bite without much provocation. I know how my throat works and we set up the loads and OAL to suit. Anyone ever notice that the 100 Berger could be loaded out longer than the factory set up? We set the jump to control the pressure. Same with the 123 Scenar. Sub a 120 SMK and then set to the lands minus a bit and the pressure will skyrocket with very little load adjustment. Tiny case = sensitive.
    http://www.65grendel.com/forum/showt...ll=1#post75339
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  6. #26
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    Quote Originally Posted by brut28481 View Post
    Agreed! Do you think I might also have a bolt face tolerance issue that also contributed to early lug failure? If so, how do I check and prevent with new new bolt in addition to dropping my load? Thanks!
    What about the possibility the guys at the factory loaded the wrong powder into the tin? Nope, don't shift blame or you will do it again. Loading too hot broke a perfectly normal bolt.

    Quickload has the safe limit at 28.5gns 8208. I get 2600fps over the chronograph with that load and that's more than enough to ring steel and punch holes in paper.

  7. #27
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    Invaluable insights, thanks again!

  8. #28
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    Quote Originally Posted by brut28481 View Post
    Agreed! Do you think I might also have a bolt face tolerance issue that also contributed to early lug failure? If so, how do I check and prevent with new new bolt in addition to dropping my load? Thanks!
    brut:

    I didn't see anything out of the ordinary with your bolt face but I am no expert on building AR-15's. NugginFutz and LRRP52 are the guys to ask about that.

    LR55

  9. #29
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    Quote Originally Posted by Klem View Post
    What about the possibility the guys at the factory loaded the wrong powder into the tin? Nope, don't shift blame or you will do it again. Loading too hot broke a perfectly normal bolt.

    Quickload has the safe limit at 28.5gns 8208. I get 2600fps over the chronograph with that load and that's more than enough to ring steel and punch holes in paper.
    More likely is the normal lot-to-lot variation of powders. SAAMI standards indicate a 4% standard deviation in pressure is to be expected with normal practices.

    My own pressure testing shows results similar to their standard.

    There’s a boatload of other factors that affect pressure, many of which involve details of one’s chamber and throat. For example, allowing the bullet to be in contact with the rifling at hammer fall can increase pressure by as much as 5ksi. I know from my own testing that allowing the case to grow in length enough to be in contact with the start of the throat can add as much as 8,000 psi. To the best of my knowledge, non-standard practices like this are not included in the SAAMI error budget.

    All of these and others can happen without one really being aware of the issue.

    So yes, heed the advice of well-seasoned handloaders and EVERY handloadong manual:

    DO NOT EXCEED MAXIMUM LOADS!
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  10. #30
    Just one observation, and it may not even be valid. I see a pretty good ring around the primer on the bolt face.

    Could just be carbon, or could be gas etching, etc.

    I only mention it as I saw that on bolts where I know I unintentionally had some hot batches. Loosened primer pockets, gas/carbon leakage, etc.

    I don't see that on my dialed in loads or rifles shooting mostly factory ammo. I'm also using AA coated bolts more now, and I don't think they mark as much.

    But thought I'd mention it, especially if you are seeing black rings around primers on the brass, loose pockets, etc.

    And I'm sure you know it, but I'll mention it as it bit me early on: You can what you would think would be safe powder loads that produce much higher pressures due to the projectile being jammed into the lands. Don't mean to insult your intelligence, it's just a learning curve I experienced a decade ago that we sometimes forget.

    I consider myself very lucky that I've not broken a bolt as I know I've loaded way hot early on. I've retired one bolt to backup status for that reason. It's about 7 lives into it's HPT/overpressure experience. :-) And I shall name him "Lucky".

    I'm largely loading for reliability and not bug-nuts accuracy, so lately I tend to go conservative in jump as I want my ammo to be usable in multiple grendels safely. My benchmark has been Hornady factory A-Max/SST. I duplicate their seating depth, and it works great. Obviously different bullets need different seating depths, but I use similar approaches even on my other loadings. (Mostly blem nosler CC)

    I don't see a big accuracy compromise, but I'm a carbine/hunting guy and the Long range guys would say I'm nuts. :-)

    All that said, if you look over a long period of time, there is not a significant pattern of grendel bolt breakage that is not either due to age/# of rounds, or one of these factors:
    • 7.62x39 bolt usage (known to be weaker). AA followed Colt's engineering experience when developing the Beowolf/Grendel, for some very specific reasons.
    • New Grendel mfg not making per spec/correct heat treat/appropriate alloy/correct lug relief
    • Hot loadings (looking for bolt gun brass signs, short throats, or not dialing in seating depth)
    • One very early wave of grendel bolts from one contract batch that went to multiple mfgs that did not have correct heat treat.

  11. #31
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    The AR15 was designed for a cartridge with a smaller case head (5.56).
    Increasing the case head diameter means pressure has to drop to maintain the same bolt face thrust.
    It is just simple physics.

  12. #32
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    Great points, longer case length is something I never considered. It's very possible I personally have some long cases that could have contributed to my boot failure. Circling back to max load for this particular round: Sierra/quickload showing max of 28.5 with Hogdon showing a max of 29.8. I would assume the pressure difference between these two loads at the upper end is pretty significant for the Grendel. Why would Hogdon have such a higher published load? What am I missing?

  13. #33
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    Great point about primer flattening not being a good indicator of high pressure with 50K pressure rounds.

    I guess velocity and maximum book loads are the only useful indicators.

    I have thought factory Wolf and Hornady factory primers looked a little too flat for me.

    My starting level handloads had the primers showing a nice radius.

    For me, it is not worth loading maximum for 100 to 200 fps more velocity.

  14. #34
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    Quote Originally Posted by brut28481 View Post
    Great points, longer case length is something I never considered. It's very possible I personally have some long cases that could have contributed to my boot failure. Circling back to max load for this particular round: Sierra/quickload showing max of 28.5 with Hogdon showing a max of 29.8. I would assume the pressure difference between these two loads at the upper end is pretty significant for the Grendel. Why would Hogdon have such a higher published load? What am I missing?
    They are using different bullets (SMK v. BTHP/TMK), different seating lengths (2.250 v. 2.260), different case manufactures (Hornady v. Lapua), different primers (205M v. WSR). There is also the question of which rifle / chamber was used in testing.

    The only common factors were the powder and bullet weight, so plenty of room for a difference in max pressure for a given charge weight.

    ETA: On a related note on Hodgon reload data:

    If you look at the label of a can of Varget, you will note that they list the load for a 168 grain .308 projectile at 46 grains. Years ago, when I first began to reload, I took this information, along with a friend's recommendation, and loaded up 100 rounds of 168 Nosler CC's in shiny new Lapua brass with 46 grains of said Varget. I had no chronograph and only my (very) untrained eye to evaluate the results.

    What I eventually came to realize was that in my particular 308, a CZ 550 w/ 26" barrel, that load was way over max pressure. It absolutely savaged the brass, caused flat primers and delivered stupid high MV's.

    I ended up having to drop a full 2 grains, down to a 44 grain charge, before pressure signs all disappeared and an accuracy node was found.

    Just because the say it's max for their setup, doesn't mean it's safe for yours.
    Last edited by NugginFutz; 12-04-2017 at 10:31 PM. Reason: Added Varget information
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  15. #35
    Quote Originally Posted by brut28481 View Post
    Great points, longer case length is something I never considered. It's very possible I personally have some long cases that could have contributed to my boot failure.
    many here more expert than I, but the two big things with Auto's and Grendel specifically that bite people:

    - Bullets jammed into lands due to seating depth/long projectile issues. Much less reduced with the 123g Hornady, etc. But in the early days certain bullets were very difficult to use. (120g hornady, etc) This effect also seen if a barrel mfg used a worn or incorrect reamer. (Much noise on this a while back with even factory ammo)

    - Case neck too long, essentially crimping the bullet when chambered. Easy fix, check/trim the brass. More rarely, neck chamber dimensions too short. Sometimes seen with inexperienced custom barrels. etc.

    As to max load, I'll defer to others. I will say I've found more usable load info from this site, worked up to carefully on my own gear, than I ever found in the loading manuals.

    That and recognizing you can't depend on reading brass like with bolt guns or 5.56 in the AR due to the bolt thrust issue. Want a bigger base? You will have higher bolt thrust. It's a tradeoff. Grendel stays inside the safe maximum for it's config, but is more dependent on proper construction/heat treat, etc.

    I would not say Grendel bolts are weak, they are not. They are, however, less tolerant of abuse/mis-mfg than less demanding cartridges (5.56 and 7.62x39 as examples).

    Myself, I'll happily take that tradeoff. The difference between cheap bolts and solid, proven ones is not enough for me to sweat. One day chasing my tail on the range and I've paid for the nominal higher cost of a good bolt.

  16. #36
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    Looks like your lug faces near the extractor are contacting the breech face more than the others.

    Maybe the breech face isn't square.

    The bolt impacts the breech pretty hard in normal operation. It gets pounded every which way but Sunday, then also endures shear forces during unlocking. For a DIY build that someone wants to run hard, I would recommend lapping the bolt lugs into the extension after the extension is trued, along with the upper. BAT Machine makes really nice extensions with tight dims, but I would still check them for square.

    As to why Hodgdon's and Sierra's data is different with 8208XBR under a 107gr SMK, the lot-to-lot powder variances and even the way they are instrumenting can contribute to the differences in peak pressure readings.
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  17. #37
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    Quote Originally Posted by LRRPF52 View Post

    Looks like your lug faces near the extractor are contacting the breech face more than the others.

    Maybe the breech face isn't square.

    The bolt impacts the breech pretty hard in normal operation. It gets pounded every which way but Sunday, then also endures shear forces during unlocking. For a DIY build that someone wants to run hard, I would recommend lapping the bolt lugs into the extension after the extension is trued, along with the upper. BAT Machine makes really nice extensions with tight dims, but I would still check them for square.
    I recall bwaites once expressing his misgivings about doing such a thing, as he was concerned about removing the hardened face of the bolt lugs.

    Your opinion on how to reconcile the two schools of thought, please?
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  18. #38
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    Quote Originally Posted by NugginFutz View Post
    I recall bwaites once expressing his misgivings about doing such a thing, as he was concerned about removing the hardened face of the bolt lugs.

    Your opinion on how to reconcile the two schools of thought, please?
    Doesn't seem to affect them with the wear that happens naturally on the backs of the lugs, which are more critical than the front.

    I made my own 6.5 Grendel lug lapping case with a spring in it.

    You pull the extractor and ejector and cycle the bolt about 100 times until you get a good lapping of the lugs to the extension.
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  19. #39
    +1!!

    Quote Originally Posted by LR1955 View Post
    Brut:

    You are still over the max recommended load. Even by .2 grains providing your scale was accurate.

    I won't belabor the point. Max out loads for a Grendel, particularly with a fast powder for that specific bullet, and you will shear lugs way before they should shear. Go over max and you will shear them sooner.

    LR55

  20. #40
    Quote Originally Posted by LRRPF52 View Post
    Doesn't seem to affect them with the wear that happens naturally on the backs of the lugs, which are more critical than the front.

    I made my own 6.5 Grendel lug lapping case with a spring in it.

    You pull the extractor and ejector and cycle the bolt about 100 times until you get a good lapping of the lugs to the extension.
    I think i have seen you post that before, with some pictures, but couldn't find it. You mind posting that again?

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