A max 123 gr with IMR8208XBR and Benchmark 2.302 inch COAL

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  • Glock357
    Bloodstained
    • Jun 2016
    • 50

    A max 123 gr with IMR8208XBR and Benchmark 2.302 inch COAL

    I am new to the Grendel so I am trying to develop an accurate load for my new Grendel built.

    I ve had a 6.8 SPC for a few years now so I already had some Hodgdon Benchmark and IMR8208XBR and I decided to try these ones first.

    Load and range data:
    Powders:
    IMR8208XBR from 27 to 29 gr in 0.5 increments
    Benchmark from 27 to 27.5 gr
    Cases: are Hornady new, resized, trimmed to 1.513 inch
    Primer: CCI41
    Bullet: Hornady A-max 123 gr.
    COAL: 2.302 inch.
    Crimp: light crimp with the Lee factory crimp.
    Velocities: (see chart for details)
    XBR: from 2443 to 2617 fps
    Benchmark: from 2501 to 2544 fps
    Temperature: 88F
    Altitude: 800ft
    Caldwell Chronograph at 10 feet away
    5 rounds of each load
    Barrel: 6.5 Grendel II 24 inch 5R, rifle length gas, no muzzle device installed.

    Notes:
    These are loaded long, but they feed nicely both in my PRI and ASC magazines (I am using 6.8 SPC mags hat I had already)
    I made a home-made distance to lands COAL guage to see how much jump I will have with this COAL and I measured 2.356 inch COAL when the bullet touches the lands giving me about 0.05 inch jump to lands.
    The result of the long COAL was that up to 28.5 gr I could hear powder rattling in the case, but not at 29 gr which means that at this COAL about 28.5 grains is the case 100% capacity for this bullet.
    As expected Benchmark was very accurate, but it's probably too fast for this round. XBR did well, but didn't impress me.


    image.jpeg
    Distance to lands COAL guage
    image.png
    Summary of hand loads. On the chart I listed the two loads, velocities and std, group size. Red means too hot, don't try.
    image.jpeg
    Best group target

    I was surprised and impressed how much faster the factory A-max load was in my Rifle: I registered 2648 fps 10 shot average with 36 std 1.25 inch at 200 yards. I would love to replicate this round!

    Now this is the first time I shoot this brand new barrel, so this was my breakin. Results might change in the future.r

    I know 28.5 gr XBR is the favorite load of many here on this forum. Is there a powder that can do better that I should try? I was thinking A2520, Hodgdon Varget or LVR?

    Here are my requirements:
    I need a powder that is slow enough but not too slow for a 24 inch barrel that is safe enough to fill 100% the case for COAL 2.302 inch, probably 29 gr extruded or 29.5 ball that is stable in the temperature range I shoot: 25F to 100F. Do you guys have any suggestions?
  • BluntForceTrauma
    Administrator
    • Feb 2011
    • 3897

    #2
    Great report, thanks! Try this thread for some load ideas.

    It seems XBR in the 65G comes into its own in the lighter weights, say 107 or lighter, and A2520, AR-Comp, or LVR are slightly more suited to 123 and above.

    BTW, you know about the Glock 32?
    :: 6.5 GRENDEL Deer and Targets :: 6mmARC Targets and Varmints and Deer :: 22 ARC Varmints and Targets

    :: I Drank the Water :: Revelation 21:6 ::

    Comment

    • Glock357
      Bloodstained
      • Jun 2016
      • 50

      #3
      Thanks for the link, I was looking at it before I started loading to see what was reasonable. I also like your original ideas for coming up with universal loads for the Grendel. In my area we are a bit limited with reloading components but I be going on a road trip soon so I ll try to stock up those powders.

      I actually have G22 with a 357 6 inch barrel that I take hunting with me. Easy conversion is such a plus

      Comment

      • Cornbread
        Warrior
        • Dec 2015
        • 288

        #4
        You might try a shorter coal. Mine likes 2.245 ish. Now that the cases are fire formed you may pick up a little velocity. Be sure and only bump the shoulders back .003-.004. New cases prob have a little excess headspace and can give false pressure signs. I am getting 2480 avg out of an 18" at 28 gr xbr.

        Comment

        • Glock357
          Bloodstained
          • Jun 2016
          • 50

          #5
          Interesting point. I resized my new cases using the Lee full size die. I ll compare the fire formed cases with the resized ones to see how much difference there is. I hope I don't have barrel head spacing issues. The Lee die is to spec because I checked the resized brass using a Wilson gauge. I will try different COAL once I get different powders. I ll save the XBR and benchmark for the 6.8 I have as they work much better there. When I fired the factory loads I was surprised to see primer cratering. I don't know if this is normal or if this is because it's a new barrel.
          image.jpeg

          Comment

          • FW Conch
            Warrior
            • Nov 2014
            • 289

            #6
            Those primers look fine to Me. All I see is 52,000psi trying to push the primer through the firing pin hole. As long as they are not pierced, I'm ok with it.

            Because of the efforts of very knowledgeable people using the latest high tech equipment, we all know that the traditional pressure signs we relied on, before that equipment was widely available, are very unreliable. Accurate pressure evaluation can only be accessed by pressure trace equipment, or chronograph, velocity, data. It seems very hard to make the brass reading hocus pocus go away? Other than that, we only know that if we are at max book loads, we are likely over max book pressure, because of the difference between the test equipment and our rifles.

            If I am in error with this line of thinking, please enlighten :-)

            Comment

            • WitchDoc
              Unwashed
              • Jun 2016
              • 19

              #7
              Originally posted by BluntForceTrauma View Post
              Great report, thanks! Try this thread for some load ideas.
              Wow, thank you for this! Much appreciated.

              Comment

              • JASmith
                Chieftain
                • Sep 2014
                • 1620

                #8
                Well said!

                Brass reading is as much an art as a science. One must look at multiple indicators, especially with the AR, to infer that loads are approaching uncomfortable levels.

                Further, it is a technique that requires a lot of practice to be useable.

                So while neither precise nor particualarly accurate, brass reading can give insight about one's load. It can also be very useful for diagnosing gas and operating system issues.

                Regarding Strain gage pressure measurements, it seems that surrogate actions are needed to take pressures. The design of the AR breech results in threads being over the body of the Grendel case. That location is where the gage must be, so we cannot get the needed measurements directly from the AR.

                Even when using a surrogate pressure rig, one needs to calibrate against pressure data. One will quickly learn why SAAMI tells ammunition makers to assume a standard deviation of 4% of the nominal pressure. My own results suggest that it is easy to see variations of 3,000 to 4,000 psi going from one load to another -- even from the same data source!
                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

                Comment

                • Glock357
                  Bloodstained
                  • Jun 2016
                  • 50

                  #9
                  Thanks very much for this input. It's very reassuring. Through my experience I gained some intuition on pressure signs using indirect measures, which is all I have to go with right now, but as you all said I have to take it with a grain of salt. I suspected hornady uses a thinner primer case than my CCI41 so I can't compare the 2. From my previous reloading experience I used velocity increase reduction as the leading indicator of over pressure, and I try to start low and increase gradually. This forum is also a very useful knowledge base for what is reasonable.

                  Comment

                  • Cornbread
                    Warrior
                    • Dec 2015
                    • 288

                    #10
                    With a round like the Grendel that operates below the pressures at which you normally start having issues with primers, you need to be careful. Watching the velocity across the chrono is a very good way to tell where you are. Once it flattens out and an increase in powder charge does not correlate to a linear increase in velocity it is wise to stop. I would try the max loads you achieved before with the resized brass and see where you are. It is important not to oversize the brass. Once you find your happy place with pressure try shortening the coal and see if the groups tighten up. Hornady is using some sort of proprietary blend of powder that's not available to the public. That is how they are getting the velocity at saami pressures.

                    Comment

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