How much jump to the lands is too much?

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  • How much jump to the lands is too much?

    In the jump and crimp thread, rasp65 posted a method to accurately measure OAL to determine how long you can seat a bullet to reach the lands. I dutifully sliced the neck of a case and peformed the measurement with four different bullets. It works quite well, and the measurements are easily repeatable to the thousandth.

    To my surprise, I discovered that with my barrel I can't get anywhere close to the lands if I seat at mag length. Here are the cartridge OAL measurements I took for each bullet to reach the lands:

    Scenar 123 - 2.342"
    Amax 123 - 2.330"
    Sierra 123 - 2.347"
    Sierra 107 - 2.351"

    I've read here on the Grendel forum that some folks have trouble finding loads that don't touch the lands, so the throat on this barrel seems excessive to me. I don't have many rounds through the barrel at this point, but the accuracy so far certainly isn't what I'd hoped, particularly for a $550 tube. Thoughts?

    Scott

  • #2
    Scott, There is a class of bullets specially designed for better performance at longer ranges. They have a lot of their length beyond where they increase in diameter to caliber and touch the lands. I believe the four you referred to are among that class of bullet.

    What you read was folks talking about was when using normal bullets, with shorter amounts of length beyond that point. I've read that it is to be expected. So far, I have only loaded hunting bullets. All of them hit the lands between 2.27 and 2.28 COL. To me, that is about as perfect as you could ask for to enhance accuracy.

    The thing I dislike the most about the AR class of rifles is the fact that most factory guns are made with a military influenced, long throat. By that I mean you can shoot them 500-1000 times without cleaning and still get a round to chamber despite lots of crud built up in that long throat. Sports shooters, which I consider myself among, usually clean their weapons more often than that, so we can be afforded a shorter throated barrel. The Grendel is the first caliber I've seen in an AR, that offered that feature. It is so nice to be able to make the decision whether to load an AR into the lands if I choose to, as opposed to not even having the ability to make that choice.

    Hoot

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    • #3
      Thanks for the reply, Hoot.

      I was curious if the throat in my barrel is longer than what one might consider "normal" for the Grendel, though mine is actually the LBC .264 chamber, not a Grendel chamber. I bought the barrel with the intent to use it as a long range target gun with the high BC bullets I mentioned, so I'm perplexed that the OALs I need to shorten the jump to the lands seem awfully long to me.

      I have another barrel inbound that I plan to use as a dedicated hunting rig, and it will have a the Grendel chamber, so it will be interesting to see how that chamber/throat difference compares accuracy-wise to my target tube.

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      • rasp65
        Warrior
        • Mar 2011
        • 660

        #4
        Scott
        Hoot is correct about your bullet choices. I was checking my data and found the 120 Nosler BT, 120SMK,and 120 Speer are all less than mag length in my rifle. BA recomends keeping them .007" off the lands. I have found the 120SMK to work the best in my rifle.

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