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Thread: Are broken bolts really an issue with 6.5 Grendel?

  1. #1
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    Are broken bolts really an issue with 6.5 Grendel?

    Would like to hear input on this if anybody has firsthand background with 6.5G and the alleged bolt issue. I'm considering a custom-built 18" 6.5G upper, but see a lot of mixed reports of broken bolts in Grendel rifles. It's hard to get the straight story. Some say this is an Internet myth, some say it USED to be an issue now it's resolved, some say that it's inherent to trying to run the 6.5G on the AR15 (their argument is that because there's simply less material there, the bolt cannot be as durable). I talked to one of the leading precision builders of the 6.5G, and when I asked about it the owner said: "We've only had 6 broken bolts reported in the last year." He was saying that as a good thing. But I was doing the math in my head, say his business built even a couple hundred 6.5G uppers/rifles in the last year (they build a lot of other things as well, so they wouldn't be cranking out just Grendel uppers exclusively). Six broken bolts out of a couple hundred is too many, IMHO. You hop into discussions at other forums like the Hide, some say they've had multiple broken bolts, some say they never have broken a bolt. The impression I have gotten--which I admit could be wrong, and I actually hope I AM wrong because I want to get this upper--is that the folks that run their 6.5G rifles hard and shoot a lot, do eventually get broken bolts regardless of the bolt's reported quality or who the builder was.

    The problem is I don't have much to go on besides the above. If anybody has more that can either confirm this issue or put the myth to rest, would like to hear it.

  2. #2
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    My 6.5G build is still in the works, so I cannot give any reliability numbers. The bolt is from JP because of a recommendation from the barrel manufacturer. What I have read indicates Alexander Arms and Maxim also make better quality bolts.

    Keep in mind that many people are shooting the steel-cased Wolf ammo in 6.5G. This probably has some affect on the percentage of broken bolts versus .223/5.56 where cheap brass-cased ammo is abundant.

  3. #3
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    Broken bolts are a fact of life with the AR-pattern design, regardless of caliber. The shear forces on the cam pin hole are just too much regardless of material and the harder the gas system runs the gun, the shorter the bolt life. It's just the nature of the beast.

    After 5000 rounds, an AR bolt is living on borrowed time. It may last 20K rounds longer, or 1 round, but the one thing history informs us is that the bolt in an AR is going to break across the bolt pin hole eventually. (this is the motivation for the various and sundry round-counting electronics being tested/added to AR-pattern systems in SOCOM and beyond).

    Failures in other areas of the bolt...e.g., the lugs, across the bolt face, firing pin hole micro-cracking, etc....are not unheard of, but most bolts don't last long enough to get to those round-counts.

    The 6.5G case head size does add some marginally greater stress on the lugs. Just like every other caliber, if fewer lugs are carrying the load (usually due to out-of-spec lug back faces or barrel extensions) the lugs that carry the load are subject to fracturing or shearing sooner rather than later. With the larger case head, the lugs are SLIGHTLY more likely to develop cracks toward the front of the lug that can migrate into the bolt face.

    It is impossible to determine when any of those issues will manifest themselves on any particular bolt without bolt-specific testing.

    That a particular maker has experienced numerous failures in the bolt they use is disconcerting. One thing not provided was round-count-at-failure, however. That info would be important.

    To me - and I have a very long history with AR-pattern systems in heavy use - an AR bolt is a maintenance item that should be replaced at 5K-ish rounds on any serious-use AR. 'Play' or 'fun' ARs are completely different and the bolts get replaced at first sign of incipient failure (usually one side or the other of the cam pin hole will develop a visible crack).

    As aways, YMMV

  4. #4
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    Quote Originally Posted by MJN1957 View Post
    Broken bolts are a fact of life with the AR-pattern design, regardless of caliber. The shear forces on the cam pin hole are just too much regardless of material and the harder the gas system runs the gun, the shorter the bolt life.
    That is true, and agree also with your other point that it would help to have more information on the failures: how many total 6.5G uppers, what loads they were running, and round counts at failure. This was a quick phone conversation about spec'ing a precision upper, so at the time I didn't think to start grilling the guy for details about the failures, now I kinda' wish I would have.

    On your point about the gas system, do the following seem like reasonable things to do to mitigate that?
    * Add an adjustable gas block and optimize for loads.
    * Use intermediate length gas (optimized to 18" barrels)
    * Run a VLTOR A5 buttstock (I already have these on my lowers)

    Those are the main things I'm aware of to try and deal with over-gassed uppers.

  5. #5
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    I always though of bolts as wear items and have a couple spares for my regular ARs (5.56, 300 Blk), but realized upon reading this thread I should grab a 6.5 bolt as a spare as well.

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