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Thread: 308 pressure in an AR-15 chamber; Ruger SFAR

  1. #1
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    Dec 2006
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    308 pressure in an AR-15 chamber; Ruger SFAR

    I have a concern regarding the design of the Ruger SFAR which uses an AR-15 size barrel but chambers a .308 cartridge.

    In my collected notes I found a post from Bill Alexander of Alexander Arms regarding the design and pressure of the 6.5 Grendel cartridge. I reproduce it complete below, but bold part for emphasis.

    Does anyone reading have the technical knowledge to analyze the radial chamber expansion of the SFAR in this way? How could the SFAR be designed "to resist creep of the threads with cyclic loading" from a 62kpsi cartridge with a larger diameter than the 6.5 Grendel? Is the breach properly sized to avoid "hard extraction and case head separation" in the SFAR, even with brass cases? Does Ruger anticipate a problem here, and is this why there are additional vent holes added through the barrel extension and upper receiver?

    Bill Alexander
    08-10-2012, 04:22 PM
    This is an interesting question.

    Grendel has the MAOP set at 50,000 psi. This was established as the ideal working load for the rifle to allow for a durability cycle that matches the existing military needs for a fleet size sample. It may be a little conservative. The maximum pressure for the round at STP is 51,800 psi. This will account for production variations, storage variations for a batch and to a certain extent a temperature/instrument wander.

    The case is developed with a 58,000 psi web. Depending upon manufacturer the offset for the case varies between 5,600 psi and 9000 psi which is an indicator of the ability of the case to operate in a automatic mechanism, which opens with some level of residual pressure.

    The rifle itself is rated to the same as the case at 50,000 psi for operation but has to also be able to fire proof loads without sustaining damage. In this instance 67,340 psi. The difference between the MOAP and the proof loading constitutes the safety factor and allows for the durability.

    In examining the system as a whole one must balance all aspects of the design. Exterior ballistics depend upon the interior ballistics which in turn must be reflected by the weapon weight and recoil. Increasing the working pressure would result in a higher Mv but it would also increase recoil. The offset might be additional weight to strengthen the mechanism but for the same weight system you then loose controllability and the ability to rapidly engage targets at close quarters. So essentially you trade a little increase in Mv for weight and speed. And in increasing the strength of the mechanism one must not just look to what the locking surfaces can resist. The interface between the barrel and the locking surfaces must be adjusted in diameter to resist creep of the threads with cyclic loading. Equally the diameter must be increased to match the radial expansion of the chamber under pressure with the onset of plastic deformation of the case. If the breach is not matched to the operating pressure then hard extraction and case head separation becomes probable. At 50,000 psi a correctly operating Grendel will require a minimum breach of 0.900" to operate with steel cases. This is more severe than brass as the brass will spring slightly more. The current thread pattern only provisions 0.736" root diameter so a true class 3 thread has to be provided to just be compatible. Interestingly the 5.56 can run too high a pressure for the breach with steel cases as it is already.

    In a weapon where the ballistics are dominated by the Mv then it becomes essential to drive the internal ballistics. For a lightweight weapon where the projectile form becomes a dominant part of its flight characteristics the small gains seen in shifting internal pressures are not as important.

    So returning to a 60,000 psi bolt, this is just a single piece of the jigsaw.
    Last edited by Disciple; 08-13-23 at 13:37.
    "We must all hang together, or, most assuredly, we shall all hang separately."

  2. #2
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    Mar 2015
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    1. Proper design of the chamber, or adjusting the case hardness (yield strength) can solve that problem.

    Last edited by lysander; 08-13-23 at 15:53. Reason: edited for errors

  3. #3
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    Mar 2015
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    Also, what is the diameter of the SFAR barrel, or what threads are they using?

    If you go from a 13/16-16 to a 13/16-32 thread you pick-up an additional 0.037" in diameter. May not sound like much, but it drops the hoop stress 19%

    If you use a 7/8-32 you're almost back to AR-10 wall thicknesses (that would make a thin extension, though [EDIT: but, should be strong enough]). They also could have used a completely proprietary thread to balance the wall thickness of the barrel and barrel extension to keep the overall diameter 1 inch.
    Last edited by lysander; 08-13-23 at 15:48.


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