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6BRX

sandwarrior
On of the things I've come up against while loading 'advanced' rounds chambered in an AR-15 is how much pressure can the AR-15 platform take? I've heard many stories about how well it will take this or that round, then find out several of 'this or that' round have caused failures.

So, the questions are these: Can an AR take the pressures of the 6BRX in all bullet weights. Can it take the pressure if you necked it up to .257? 6.5mm? .270? 7mm?

Any thoughts?

Also, what are the necessary adjustments to the Barrel extension, bolt/bolt face?
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5 comments

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  • tsr1965
    Todd,

    I would assume the pressures would still remain the same for the parent case. However, going to a larger diameter bullet, would increase the thrust. Just like going to a larger diameter case head will increase the thrust, for a given pressure range, given that the bullet size also increases. That is why TC Arms, would never chamber a Contender on the 308/30-06 case head.

    That said, I know that Olympic chambers in the WSSM chamberings, which have both higher pressures, and increased case head surface area. Then there is a shop in Indiana who takes those up all the way to at least 0.358" diameter, to meet the hunting laws there.

    No, that does not give you a definnative answer about the "pressure", but it will give you some idea of the thrust. Also bear in mind, that I have heard some bad things about the AR-10 platforms, in use in the sandbox, that after a couple thousand rounds, they snap off the front pin holes in the reciever. Not sure if that is truth or fiction, but if it is true, it is trouobling about some of these rounds being introduced today.

    Best
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  • nononsense
    sandwarrior,

    Pressure is pressure.

    How that pressure is distributed is the test of the parts and assembly. The design of the parts should exceed the required pressure and the resulting thrust.

    I'm running an Armalite upper and lower with an AR Precision extension and bolt for the safety factors of their design. So far it runs like a top with the 105 gr. VLDs that I'm testing. Unfortunately, it's not just a straight forward 'kit gun' assembly like so many are used to putting together. There has been a substantial amount of tuning and adjusting to get the performance I need for a couple of the competitions coming up.

    Magazines are a nuisance. They are either expensive and still require adjustment or they are reasonable and still need adjustment. I think the best bet is to make a new follower at the minimum but in reality, a better designed magazine would be the best bet.

    Best.
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  • sandwarrior
    quote:Originally posted by nononsense
    sandwarrior,

    Pressure is pressure.

    How that pressure is distributed is the test of the parts and assembly. The design of the parts should exceed the required pressure and the resulting thrust.

    I'm running an Armalite upper and lower with an AR Precision extension and bolt for the safety factors of their design. So far it runs like a top with the 105 gr. VLDs that I'm testing. Unfortunately, it's not just a straight forward 'kit gun' assembly like so many are used to putting together. There has been a substantial amount of tuning and adjusting to get the performance I need for a couple of the competitions coming up.

    Magazines are a nuisance. They are either expensive and still require adjustment or they are reasonable and still need adjustment. I think the best bet is to make a new follower at the minimum but in reality, a better designed magazine would be the best bet.

    Best.




    And you are running the 6BRX? or an improved version of the 6BR? I was in contact with a member of another forum who shoots one and he told me the cases volume is 43 gr. vs. my measured Grendel volume of 33 gr. That's a pretty impressive jump. Especially for a case short enough to stick some high BC bullets in.

    Pressure does have it's various issues. As tr1965 noted, when it comes in the form of bolt thrust is when it becomes a more limiting factor. Not that if you are not seeing bolt thrust (like loading for Ackleys and WSSM's) that too much pressure is okay. Also, front pin bosses(?) breaking off sounds like a vibration problem. Maybe caused by too high of pressure. Anytime two pieces get smacked together because of the violence of a shot being fired, it's going to cause something to break eventually. As nn noted his upper and lower are matched. That would need to happen in this platform especially as hardened aluminum alloys tend to get very brittle when subjected to severe vibrations.

    I can fully understand the magazines being an issue as of right now. That issue would only go away when dedicated magazines were made for the round considered to be chambered. WSSM mags had to come out before they fed well. Same with the Grendel and the 6.8, in spite of feeding okay with a different follower, still needed it's own mag requirements to feed reliably.

    I guess my whole thing boils down to just how tough the joint is between the barrel/extension and upper receiver is and how to improve upon that so that the bolt when locked in battery is strong enough to reliably take maximum pressures. And yet still operate in the same gas system. Add to that the possible necessity of a gas/piston system so that slower powders could be used. Instead of it just being an option now.

    Just thoughts rolling through my head as my second Grendel came to fruition[:D] ...and now there's something bigger and better on the horizon.[}:)][:D]

    Thanks again for your replies.
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  • nononsense
    sandwarrior,

    This is indeed a 6BRX as originally designed. I finally got this one up and running under peak pressure for several magazines. It wasn't a problem with the rifle, it was my lack of time and getting the parts made then fitted. But this is a specialized rifle, not one you'll find at your local range. It is designed with a 26" barrel and as light as possible, especially considering that the scope weighs nearly 2 lbs alone.

    Best.
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  • sandwarrior
    nononsense,

    Best of luck to you and I hope thing does you well at the upcoming competition. I'm interested to hear how it fared for you when it's done.
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