Grendel Pressure?

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  • Grendel Pressure?

    Now that SAAMI has adopted the Grendel, what did they list as the safe operating pressure? I have read, 45K psi, 50K psi and 52K psi.

    Lets assume it's 50K psi. Why such a low pressure rating compared to other common AR rounds like the 5.56(62K) and the 6.8 SPC (58K).

    I know the rim diameter of the Grendel is .018 larger than the 6.8 which may account for some of the reduced pressure rating, but what else? After all the 6.8 has a rim .044 larger than the 223 and runs at a higher pressure (+ 3K psi).

    I guess I'm just curious, How was the 50K psi rating determined and with what type of equipment and or labs?

    Thanks

  • #2
    A partial answer: I do not know what procedures Alexander Arms used to set their pressure for SAAMI, but do know that their numbers are consistent with my calculations.

    The AR-15 bolts were originally designed to support the pressures and diameter of the 5.56 NATO round. Both the 6.8 and Grendel have larger inside base diameters, so the operating pressures are necessarily lower. The reduction is not as obvious in the 6.8 SPC because the diameters are closer to the 5.56.

    What does this mean? Look at the largest inside diameter of the Grendel case (not the rim diameter). Multiply the area resulting from that diameter times the maximum pressure. The answer is the nominal bolt thrust the locking lugs must support.

    In practice, the bolts are designed with some margin for error, but the closer one gets to the nominal shear stress of the lugs, the faster damage accumulates in the metal. When the damage reaches a certain level, the lugs fracture. We don't know with precision how many hits a bolt will take at a given pressure, but do know that below a certain level, the bolt life is effectively infinite.

    The 50 ksi was set, I as I understand, to get that long life and coincidentally gives kind-of close to the same thrust as a .223 Rem/5.56 would see when running at maximum pressure. Running higher pressures increases the chances that your bolt will fail. We don't know how many shots will be needed but do know that, on average, bolt life rapidly decreases as the pressures go up.
    Last edited by Guest; 12-06-2011, 03:33 PM.

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    • Bill Alexander

      #3
      Operating pressure is 50,000 psi

      It is derived from the requirement for a complete durability cycle within the AR weapon system. Determination was from old fashioned fatigue analysis of the design and trials of the weapons. Ultimately the fielded rifles have the biggest effect as the fleet durability is what matters.

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      • #4
        All well and good. Mr Alexander engenered a cartridge so it would be safe in the one rifle he thought it would be used most with- the AR. And as I remember he thought it was probaby unsafe to use that cartridge in the Contender (a bolt thrust issue if I remember). But I have never seen any thoughts on safe pressures if used in a bolt action rifle. And someday I will probably look into getting a Grendel chambered in a falling block action, it would be nice to know safe pressures.

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        • sneaky one
          Chieftain
          • Mar 2011
          • 3077

          #5
          Bill you stated long ago from your original testing- ( 65k press. full auto loads= After many rounds-- 1 lug broke off, opposite the extractor- yet- after the clean up- the weapon was still good-go ), I remember reading it, I think it was on an older posting-= old forum, or maybe from the stan christ article ?? Q.= is that an old bolt design, or what's in our AA., uppers we have now? Don't worry, we back off at least .5 grn. once we hit the press.== primer getting larger and flatter. This has been an easy cartridge to load, thank you for this way cool round, as it replaced my .308.

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