Broken Bolts?

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  • bj139
    Chieftain
    • Mar 2017
    • 1968

    #31
    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.

    Comment

    • brut28481
      Warrior
      • Mar 2016
      • 117

      #32
      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?

      Comment

      • bj139
        Chieftain
        • Mar 2017
        • 1968

        #33
        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.

        Comment

        • NugginFutz
          Chieftain
          • Aug 2013
          • 2622

          #34
          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, 09:31 PM. Reason: Added Varget information
          If it's true that we are here to help others, then what exactly are the others here for?

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          • pinzgauer
            Warrior
            • Mar 2011
            • 440

            #35
            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.

            Comment

            • LRRPF52
              Super Moderator
              • Sep 2014
              • 8569

              #36


              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.
              NRA Basic, Pistol, Rifle, Shotgun, RSO

              CCW, CQM, DM, Long Range Rifle Instructor

              6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

              www.AR15buildbox.com

              Comment

              • NugginFutz
                Chieftain
                • Aug 2013
                • 2622

                #37
                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?
                If it's true that we are here to help others, then what exactly are the others here for?

                Comment

                • LRRPF52
                  Super Moderator
                  • Sep 2014
                  • 8569

                  #38
                  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.
                  NRA Basic, Pistol, Rifle, Shotgun, RSO

                  CCW, CQM, DM, Long Range Rifle Instructor

                  6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

                  www.AR15buildbox.com

                  Comment

                  • terrywick4
                    Warrior
                    • Sep 2014
                    • 181

                    #39
                    +1!!

                    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

                    Comment

                    • keystone183
                      Warrior
                      • Mar 2013
                      • 590

                      #40
                      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?

                      Comment

                      • NugginFutz
                        Chieftain
                        • Aug 2013
                        • 2622

                        #41
                        Originally posted by keystone183 View Post
                        I think i have seen you post that before, with some pictures, but couldn't find it. You mind posting that again?
                        I believe this was it...

                        I bought a blem Spikes lower at least 5 years ago, and bought a RRA NM 2 stage trigger and LPK. I bought it with the intention of building a Grendel, but have never been able to find an upper or barrel/bolt combo at the gun shows or found them in stock online. I had found and saved an article on the Snipershide forum years
                        If it's true that we are here to help others, then what exactly are the others here for?

                        Comment

                        • LRRPF52
                          Super Moderator
                          • Sep 2014
                          • 8569

                          #42
                          Originally posted by NugginFutz View Post


                          That's it.

                          Just make sure to clean out all the lapping compound when you're done.

                          Better to do it before final assembly with just your barrel in the upper held in place with the nut, nothing else.

                          Strip down the bolt with a bolt disassembly tool, reassemble it with cam pin and firing pin, FPRP into the carrier, and cycle the action a bunch of times on a lower with a buffer and spring, no FCG.
                          NRA Basic, Pistol, Rifle, Shotgun, RSO

                          CCW, CQM, DM, Long Range Rifle Instructor

                          6.5 Grendel Reloading Handbooks & chamber brushes can be found here:

                          www.AR15buildbox.com

                          Comment

                          • keystone183
                            Warrior
                            • Mar 2013
                            • 590

                            #43
                            Thank you very much!

                            ETA....Man that was a while ago! I have a good memory...

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