Share your max. C.O.A.L.

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  • Share your max. C.O.A.L.

    Hi there,

    I started a very expensive Grendel project in Germany, see this thread for details:

    My first 6.5mm Grendel Project - after 3 .223 Rem AR-15s

    The rifle is built up and in testing the last weeks and I am facing accuracy problems, with Hornady factory ammunition as well as with reloaded ammunition. We tried different loads with VV N140 as well as XBR 8208, CFE223, IMR4895,...we are only using the finest reloading equimpent and components like Lapua Scenar 123 gr, Hornady 123 A-MAX and 123 & 120 gr Sierra MK bullets. The results are rather dissapointing. The best group (100m / 5 rds) we were able to produce was about 28 mm / 1.10" but it seemed like a statistical error - the average group size of app. 30 different loads was about 44 mm / 1,732" for a 5 shot group.

    I started to examine the gun carefully and excluded all uncertainties like scope, rail, trigger,...until I came to the chamber dimensions and the rotation free flight of my setup. I measured the max. C.O.A.L. with this tool: Hornady OAL Gauge and found out, that I am able to load a max. C.O.A.L. of about 61.18 mm / 2.408" (measured with a Sierra Matchking HPBT #1727).

    I would like to know from you if this rotation free flight is normal for a Grendel chamber or rather unusual and resulting in loss of accuracy. What are your max. C.O.A.L.s measured with the mentioned tool and what precision are you able to produce with loads when you load a C.O.A.L. of about 57,4 mm / 2.260"?

    Thanks for your help!
    Last edited by Guest; 07-02-2014, 06:44 AM.
  • cory
    Chieftain
    • Jun 2012
    • 2985

    #2
    Where did you get the barrel. With that COAL I suspect there is no compound throat. I'd bet your chamber is out of spec.
    "Those who sacrifice liberty for security, deserve neither." Benjamin Franklin

    Comment


    • #3
      It is a Lothar Walther Barrel from their German Headquarters

      What max. C.O.A.L. should I expect from a correct manufactured chamber? Could you describe the compound throat on the CIP sheet. Is that dimension "G" in the CIP sheet?
      Last edited by Guest; 07-02-2014, 01:27 PM.

      Comment


      • #4
        Was your barrel made in the USA or in Germany? That would make a difference on whether the reamer needed to be cut to the SAAMI or the CIP specifications. Both appear to specify a 0.5 degree taper for the freebore although I admit the CIP drawing is difficult to intrepret but likely straightforward for reamer makers on the continent.

        I was not able to get to the CIP Appendix CR-1 where tolerances are specified, but the SAAMI tolerances allow for the reamer to be as much as .002" (0.05 mm) larger, but no smaller in diameter, than the nominal dimensions on the drawing. That tolerance, coupled with the 0.015 length tolerance, allows as much 0.13" (3.3 mm) variation in maximum overall length.

        The 123gr SST in a spec chamber should allow a maximum COL of between the factory length of just under 2.245" which makes the maximum for that bullet about 2.38" in a chamber meeting the specifications. The SST and the MatchKing do not have the same nose shape and I don't have MatchKings to check with, but can you do the length measurement with the 123 gr AMAX which is very close in shape to the 123gr SST.
        Last edited by Guest; 07-02-2014, 02:25 PM.

        Comment

        • LR1955
          Super Moderator
          • Mar 2011
          • 3355

          #5
          Originally posted by Kohoki View Post
          Hi there,

          I started a very expensive Grendel project in Germany, see this thread for details:

          My first 6.5mm Grendel Project - after 3 .223 Rem AR-15s

          The rifle is built up and in testing the last weeks and I am facing accuracy problems, with Hornady factory ammunition as well as with reloaded ammunition. We tried different loads with VV N140 as well as XBR 8208, CFE223, IMR4895,...we are only using the finest reloading equimpent and components like Lapua Scenar 123 gr, Hornady 123 A-MAX and 123 & 120 gr Sierra MK bullets. The results are rather dissapointing. The best group (100m / 5 rds) we were able to produce was about 28 mm / 1.10" but it seemed like a statistical error - the average group size of app. 30 different loads was about 44 mm / 1,732" for a 5 shot group.

          I started to examine the gun carefully and excluded all uncertainties like scope, rail, trigger,...until I came to the chamber dimensions and the rotation free flight of my setup. I measured the max. C.O.A.L. with this tool: Hornady OAL Gauge and found out, that I am able to load a max. C.O.A.L. of about 61.18 mm / 2.408" (measured with a Sierra Matchking HPBT #1727).

          I would like to know from you if this rotation free flight is normal for a Grendel chamber or rather unusual and resulting in loss of accuracy. What are your max. C.O.A.L.s measured with the mentioned tool and what precision are you able to produce with loads when you load a C.O.A.L. of about 57,4 mm / 2.260"?

          Thanks for your help!
          With the 120 Match King and any safe load, at about any OAL (including 2.260), with both types of chamber, somewhere between 1/2 and 3/4 minute regularly. If it goes over one minute, it was me and not the equipment.

          What is 'rotation free flight'?

          LR55

          Comment

          • NugginFutz
            Chieftain
            • Aug 2013
            • 2622

            #6
            Sounds like another term for jump or free bore. Rotation-free flight. Flying w/o rotation induced by the rifling.

            Or not...
            If it's true that we are here to help others, then what exactly are the others here for?

            Comment

            • VASCAR2
              Chieftain
              • Mar 2011
              • 6219

              #7
              A few things which other posters have done to improve accuracy of their 6.5 Grendel's is to use a lapping tool to ensure the front the the upper receiver is square. They are also bedding the barrel or ensuring a good fit between the barrel and upper. They also use blue Loctite on the gas blocks.

              I used a Hornady tool to measure the COAL in a 20" Shilen 6.5 Grendel, my Son in Laws Lothar Walther 22" 6.5 CSS and my 16" J&T Distributing 6.5 Grendel which is a Chrome Moly Shaw barrel.

              With 123 grain SMK Shilen 2.230, L/W 2.304, J&T 2.309

              With 123 grain AMAX Shilen 2.296, L/W 2.275, J&T 2.267

              Comment


              • #8
                Kohoki:
                for your reference, my barrel has an SAAMI grendel chamber. Here's some measurements with different bullets: (sorry, all measurements are in inches!)
                123 amax- 2.297'' OAL to lands 1.686'' to ogive touching lands
                123 sst 2.253'' OAL 1.679'' to ogive
                123 smk 2.327'' OAL 1.747'' to ogive
                I load 123 Amaxes to 2.258'' and can get 1/2 moa or less with 4 different powders.
                I load 123 sst's to 2.245'' and get just over 1/2 moa .
                sorry, I don't have any matchkings on hand to measure for you- I tried 123 smk's a while back and had poor results.
                Keep in mind these results are from sitting at a bench with a rifle rest and also a good trigger .
                Sounds like your chamber has a long leade before the rifling. You could load rounds longer to get better accuracy, but then the rounds won't fit in your magazine.
                If you can't get the 123 Amax to shoot around 1/2 moa, with all the money you've invested in this project, I would get a barrel with a proper chamber- the barrel is the heart of the rifle.
                Hope this helps you. Don't give up.
                -mike

                Comment


                • #9
                  Thank you guys for your first answers. I had a talk with LW today, they rechecked their inspection certificate of the used reamer and assured that the chamber has been manufactured based on SAAMI specification (they are using an US reamer).

                  I just got curious after facing wide groups and measuring a 61,18mm max. COAL with that Hornady gauge.

                  CIP Datasheets:

                  Last edited by Guest; 07-02-2014, 04:23 PM.

                  Comment

                  • cory
                    Chieftain
                    • Jun 2012
                    • 2985

                    #10
                    If they used a PTG reamer, I'd still be willing to bet it's not SAAMI Spec.
                    "Those who sacrifice liberty for security, deserve neither." Benjamin Franklin

                    Comment


                    • #11
                      Update:
                      I will get a new barrel, dimension "G" will corrected and put to a min. length of 7,15 mm.

                      Comment


                      • #12
                        My problem with the Hornady (Stoney Point) OAL tool is that it does not work off of a true chambering depth of the cartridge after an inertia-based bolt closure.

                        What I mean is that when you let the BCG chamber a cartridge, it seats it deeper in the chamber than when you use the Stoney Point gauge. As such, you will get different COL results with the tool, giving you the impression you can load longer than you actually can.

                        I therefore use my chamber and black ink on the projectiles, starting with long OAL, then seat it deeper incrementally until there is no longer contact with the lands.

                        Comment


                        • #13
                          I've shot my 264LBC with 123 A-max's loaded to a COL 2.245 and it still shoots 3/4 inch or better with a .110 jump using 27.7 grains AR-comp.
                          The same handload is next to or lightly into the lands with my Grendel barrel that many sent in to have the throat scraped.
                          It has shot half inch with several loads.
                          It's easy to load within 10 thou off the lands with the short throat.
                          With the 264LBC it doesn't matter if the COL is out to 2.290 or 2.245 it shoots sub MOA.
                          Something must be off with your barrel if it won't shoot the 123 A-max under an inch at 100 meters.
                          Maybe it would be better to get a barrel from Lilja or PF and get a refund on your barrel if you get better results.

                          Comment


                          • #14
                            Organized a Manson rougher and a finisher, the new barrel will receive a new chamber with a total new reamer. 1000 Thanks to Dave Mason who was very very supportive and the ultrafast delivery of this tools to Germany!

                            Comment

                            • cory
                              Chieftain
                              • Jun 2012
                              • 2985

                              #15
                              Looking forward to seeing your results.
                              "Those who sacrifice liberty for security, deserve neither." Benjamin Franklin

                              Comment

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