Broken Bolts?

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  • brut28481
    Warrior
    • Mar 2016
    • 117

    #16
    Receiver was lapped before I installed barrel so should be square. Round count is somewhere around 700. +/- 70% with CFE & 123-Amax and the rest with XBR & 107. I had laddered from 28.5-30, 0.3gr increments and the velocity change was very consistent with 17-20 FPS change per ladder. I load at tad longer than factory with the ASC mags... Bolt Pic

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    • Kilco
      Chieftain
      • Jan 2016
      • 1201

      #17
      My Odin Works bolt took over 1300 rounds all 123s with a max charge and Probably 200 rounds of factory stuff and never missed a beat.

      New owner is running wolf and Hornady factory stuff through it and has had no complaints.

      I should Probobly call him and tell him it will break any day now...

      Comment

      • LR1955
        Super Moderator
        • Mar 2011
        • 3355

        #18
        Guys:

        Here is a very simple fact. If you think for a second that you will get away with overloading a Grendel like we could a .308 for example, you will shear lugs and eventually weaken the receiver enough that it will fail. This isn't one of these 'maybe' things. It will happen. The metal does not care what you thought when you looked at your brass and didn't see signs of pressure. Stress it past its limits over and over again and it will fail.

        Also, shoot a diet of max loads with your Grendel, particularly if you are using a fast powder for your bullet weight, and you will shear lugs far sooner than you would like. And sorry guys, shearing lugs within a couple thousand rounds is not 'normal'. The Kool Aid must be getting more potent.

        I learned this lesson when the Grendel first came out. I didn't blow a upper apart but sheared lugs right and left until I learned that the Grendel is not a competitive High Power Cartridge.

        LR55

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        • pinzgauer
          Warrior
          • Mar 2011
          • 440

          #19
          Underscoring the above points, traditional brass visual pressure signs don't occur until you are well above Grendel safe pressures. (Flattened primers, smears, etc)

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          • brut28481
            Warrior
            • Mar 2016
            • 117

            #20
            I appreciate the wisdom & experience with this round!

            Comment

            • brut28481
              Warrior
              • Mar 2016
              • 117

              #21
              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, 12:00 AM.

              Comment

              • LR1955
                Super Moderator
                • Mar 2011
                • 3355

                #22
                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

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                • brut28481
                  Warrior
                  • Mar 2016
                  • 117

                  #23
                  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!

                  Comment

                  • NugginFutz
                    Chieftain
                    • Aug 2013
                    • 2622

                    #24
                    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.

                    If it's true that we are here to help others, then what exactly are the others here for?

                    Comment

                    • NugginFutz
                      Chieftain
                      • Aug 2013
                      • 2622

                      #25
                      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.
                      If it's true that we are here to help others, then what exactly are the others here for?

                      Comment

                      • Klem
                        Chieftain
                        • Aug 2013
                        • 3506

                        #26
                        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.

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                        • brut28481
                          Warrior
                          • Mar 2016
                          • 117

                          #27
                          Invaluable insights, thanks again!

                          Comment

                          • LR1955
                            Super Moderator
                            • Mar 2011
                            • 3355

                            #28
                            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

                            Comment

                            • JASmith
                              Chieftain
                              • Sep 2014
                              • 1620

                              #29
                              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.
                              DO NOT EXCEED MAXIMUM LOADS!
                              shootersnotes.com

                              "To those who have fought and almost died for it, freedom has a flavor the protected will never know."
                              -- Author Unknown

                              "If at first you do succeed, try not to look astonished!" -- Milton Berle

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                              • pinzgauer
                                Warrior
                                • Mar 2011
                                • 440

                                #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.

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