Cycling issues in new Grendel build, please help!

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  • puncher71
    Bloodstained
    • Jan 2017
    • 25

    Cycling issues in new Grendel build, please help!

    Hello, I recently finished a 6.5 upper build with 20" barrel, rifle length gas. It shoots very well (sub MOA) so far, but it is basically a single shot. I am having issues with cycling. Half the time it wont cycle the bolt far enough to eject the spent casing at all. Sometimes casing is caught and smashed by the bolt or it barely ejects the casing a few inches from rifle and wont pick up nect round. It NEVER locks back on last round. I checked and double checked gas port alignment. With the dimpled barrel its almost impossible to mis-align. I am running it on a lower receiver that has functioned flawlessly with pistol, carbine and mid-length gas systems but never ran a rifle length gas before. Is the rifle length gas my problem? Do I need to tune the buffer/spring? Im not sure what weight buffer it is. Ive tried weighing it on my reloading scales but its too heavy to register. Im gonna try and weigh it to figure out what I have. I am running factory Hornady 123 SST ammo through it. Any suggestions?
  • Bwild97
    Warrior
    • Jan 2015
    • 217

    #2
    Did you install an adjustable gas block?

    Comment

    • puncher71
      Bloodstained
      • Jan 2017
      • 25

      #3
      No sir, barrel was custom made by Dead Shot Barrels and came with low profile .750 gas block.

      Comment

      • Rickc
        Warrior
        • Aug 2016
        • 311

        #4
        You do have your bolt carrier well lubed?

        Get you some canned air. Drop the lower and flip the upper upside down where you can see the gas tube. Stick the tube that comes with the canned air.in the muzzle and Use you finger to s seal it up. Trigger the canned air and see what kind of flow you have from the gas tube. Easiest with 2 people.

        Some uppers just need a few mags cycled.through to break in. Rough chambers and.throats. do a search on this forum and you will see it is a common problem with new grendel barrels.

        Comment

        • VASCAR2
          Chieftain
          • Mar 2011
          • 6219

          #5
          The buffer tube length carbine versus rifle length buffer tube determines the recoil spring and buffer. If your using a carbine buffer tube and spring with a carbine buffer it should work with a 20" rifle gas system 6.5 Grendel barrel. It sounds like your rifle is under gassed. When I built my 20" 6.5 Grendel I had to open the adjustable gas block a lot even with a H carbine buffer. If your lower has a heavier buffer like an H-2 or H-3 this could contribute to the malfunctions your experiencing.

          Here is a link to a short article on AR-15 buffers and springs. Really helps to know for sure what you have installed to determine the next best course of action.

          Last edited by VASCAR2; 02-26-2017, 11:42 AM.

          Comment

          • just_john
            Chieftain
            • Sep 2012
            • 1565

            #6
            Have you verified that, in fact, the dimpling under the gas block really is in the right place? Use the gas block for a "straight edge" and back it up until the edge of it goes across the center of the gas hole and then verify that it also goes across the center of the dimple. Then, be sure that the dimple is on the exact opposite side ( imagine that a hole were drilled all the way thru ).

            Comment

            • ricsmall
              Warrior
              • Sep 2014
              • 987

              #7
              Originally posted by puncher71 View Post
              Hello, I recently finished a 6.5 upper build with 20" barrel, rifle length gas. It shoots very well (sub MOA) so far, but it is basically a single shot. I am having issues with cycling. Half the time it wont cycle the bolt far enough to eject the spent casing at all. Sometimes casing is caught and smashed by the bolt or it barely ejects the casing a few inches from rifle and wont pick up nect round. It NEVER locks back on last round. I checked and double checked gas port alignment. With the dimpled barrel its almost impossible to mis-align. I am running it on a lower receiver that has functioned flawlessly with pistol, carbine and mid-length gas systems but never ran a rifle length gas before. Is the rifle length gas my problem? Do I need to tune the buffer/spring? Im not sure what weight buffer it is. Ive tried weighing it on my reloading scales but its too heavy to register. Im gonna try and weigh it to figure out what I have. I am running factory Hornady 123 SST ammo through it. Any suggestions?
              Puncher

              Take your GB off and measure gas port with appropriate size drill bit. It should be .094". If it's smaller your probably under gassed, larger and your over gassed. If you are running rifle length gas you need to be running a rifle weight buffer(5.2 oz). Every rifle length grendel I've built runs either a true rifle buffer or rifle weight/carbine length and they run perfectly. Try this out and keep us updated.

              Richard
              Member since 2011, data lost in last hack attack

              Comment

              • bigdaddyflo
                Bloodstained
                • Dec 2016
                • 97

                #8
                Gas block alignment might be an issue. Just because it came dimpled, doesn't always mean it is dimpled "correctly". I use a home made version of these to align my gas blocks - Hb Industries GBA Gas Block Aligner for .22 Caliber - makes gas block alignment almost foolproof.

                Comment

                • puncher71
                  Bloodstained
                  • Jan 2017
                  • 25

                  #9
                  Originally posted by Rickc View Post
                  You do have your bolt carrier well lubed?

                  Get you some canned air. Drop the lower and flip the upper upside down where you can see the gas tube. Stick the tube that comes with the canned air.in the muzzle and Use you finger to s seal it up. Trigger the canned air and see what kind of flow you have from the gas tube. Easiest with 2 people.

                  Some uppers just need a few mags cycled.through to break in. Rough chambers and.throats. do a search on this forum and you will see it is a common problem with new grendel barrels.
                  Yes sir, bolt is lubed up?
                  In regards to air testing, do I need to seal chamber for proper flow through gas tube?

                  Comment

                  • puncher71
                    Bloodstained
                    • Jan 2017
                    • 25

                    #10
                    Originally posted by just_john View Post
                    Have you verified that, in fact, the dimpling under the gas block really is in the right place? Use the gas block for a "straight edge" and back it up until the edge of it goes across the center of the gas hole and then verify that it also goes across the center of the dimple. Then, be sure that the dimple is on the exact opposite side ( imagine that a hole were drilled all the way thru ).
                    Everything checks out with gas port/gas block alignment.

                    Comment

                    • puncher71
                      Bloodstained
                      • Jan 2017
                      • 25

                      #11
                      Originally posted by ricsmall View Post
                      Puncher

                      Take your GB off and measure gas port with appropriate size drill bit. It should be .094". If it's smaller your probably under gassed, larger and your over gassed. If you are running rifle length gas you need to be running a rifle weight buffer(5.2 oz). Every rifle length grendel I've built runs either a true rifle buffer or rifle weight/carbine length and they run perfectly. Try this out and keep us updated.

                      Richard

                      I am contemplating going to a fixed rifle stock anyways and so will probably be getting the rifle tube/buffer/ spring.

                      Comment

                      • puncher71
                        Bloodstained
                        • Jan 2017
                        • 25

                        #12
                        Theoretically, if system is "short stroking" as it seems mine is, do you generally need a lighter or heavier buffer?

                        Comment

                        • rwh
                          Warrior
                          • Jun 2014
                          • 188

                          #13
                          I just had this same experience installing a 28 inch barrel with an extended length gas tube on a previously functioning rifle. I don't think an adjustable gas block will help here since a standard gas block should be about the same as an adjustable block adjusted for maximum gas. In my case I was able to get the rifle to cycle by switching to a JP silent spring and going with the lightest spring they make (the white one). That got the rifle working and cycling with a magazine but still not locking back. I was using 108 gr scenars with VIHT N133 because I have a couple thousand of them and they shoot really well. I expect that the rifle will work just fine with 130 gr berger hybrids and LeverEvolution.

                          I hesitate to drill a larger gas port. However if all else fails that is something to consider if you have tried lighter buffer and lighter springs and the rifle still won't lock back.

                          Comment

                          • A5BLASTER
                            Chieftain
                            • Mar 2015
                            • 6192

                            #14
                            I can only speak for my setup but a adjustable gas block even on a rifle length gas system will be lower then with a standard non adjustable gas block on the same length gas system.

                            Proved this to my self on two diffrent barrels both with rifle length gas, I also prefer fixed stocks over the adjustable kind and a scs from JP is the cats azz.

                            Comment

                            • puncher71
                              Bloodstained
                              • Jan 2017
                              • 25

                              #15
                              I've done a lot of research online and performed many tests trying to diagnose the problem. Manual cycling works flawlessly, so I don't think extractor or ejector is the problem. I pulled the gas block and double checked alignment. It seems perfect with a nice even carbon "tattoo" around the barrel port. Port is .109. I hooked air up to the barrel, plugged chamber and got good air flow through gas tube. Checked gas key and bolt rings for leakage. All good there. I'm thinking/hoping it's a buffer/ spring issue. I'm going to put a rifle length stock with rifle tube, buffer and spring so hope that does it. I'll keep you posted. Keep suggestions coming!

                              Comment

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