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Thread: HPT and MPI: still viable and necessary or outdated bureaucracy?

  1. #91
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    Quote Originally Posted by rob_s View Post
    What kind of dimensional tolerance are we talking about here? Obviously less than can be seen by the naked eye. and what are the consequences of such an out of tolerance bolt within that spectrum of measurement?
    A cam pin is specced to have a certain diameter, +- 0.00025 (this is about 1/10 as thick as a hair). Whoever decided on that, decided it was needed for some reason. Too large and it may bind. Too small, and it may be sloppy. The end result may be excessive wear and/or unreliable function. When it was time for us to select cam pins, we compared many - and the Colt ones were in spec. The others - not so much. Some were not the right hardness. Some where over-sized. Some were missing the required dry-film lube. The dry film lube is a whole -nother ball of wax because it has its own tolerance for application. So there is tolerance stack, which makes it harder to get into the +- 0.00025 range. Our solution was to make them to a drawing rather than buy them by name from an AR parts vendor. This means, we can check them to the drawing and reject the ones outside the spec. If a company were to just order 'cam pins' then they would have a hard time rejecting any outside the +- 0.00025 range because they have no drawing they can point to as to why they are rejecting it. We also then did not use regular dry-film lube but hardened electroless nickel. This coating has the lubricity but does not wear as much, and can be applied very evenly, leading to fewer parts to reject. So yes, we stick to the mil spec in general, but will deviate when improvements can be made - such as this coating (or for example nitrided barrels rather than chrome lined).

    So... are your cam pins in spec dimensionally? There are plenty out there which are not.

  2. #92
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    Quote Originally Posted by orionz06 View Post
    A bolt that is the right material and passes the HPT/MPI but is not within the specified dimensions. How many times have we heard or seen a thread where a bolt wouldn't fit?

    I don't think anyone is saying bolts don't break, but good bolts with good testing procedures seem to last a lot longer, coincidence?
    Parts may 'fit' but still be out of spec. Out of spec parts is why a headspace check is necessary. ARs are designed to just be made and put together and pass headspace. There should be no reason to buy a barrel that comes with a matching bolt 'headspaced' to the barrel if all tolerances are held correctly.

    As for why bolts break - a lot of them are made from inferior materials. 8620, 9310 are two examples. Or the heat-treat may be incorrect. Or an incorrect radius tool may have been used.

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    I don't have the AR- history-fu to answer this question, but I wonder if HPT-MPI was specified because of:

    a: The revolutionary nature of the AR system and materials at time of introduction.

    and/or

    b: The proliferation of MPI in aviation at the time the AR system was developed, and the fact that its development was sanctioned by an aviation-based company.

    Love this discussion, btw.
    Last edited by 120mm; 05-18-11 at 08:28.

  4. #94
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    Quote Originally Posted by MistWolf View Post
    Each barrel will only survive a finite number of proofs before it does fail. However, with modern steel blending techniques, proof testing is no longer the imperative it once was.
    Excellent post. One of our engineers has personally fired thousands of proof rounds through a modern handgun. He did it hand-held. The barrel never failed. And certainly that pistol needed to be proof tested when it was under initial development, to prove out the design. It is safe to say that this pistol design did not need to be proof tested for every one.

  5. #95
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    Quote Originally Posted by rsilvers View Post
    A cam pin is specced to have a certain diameter, +- 0.00025 (this is about 1/10 as thick as a hair). Whoever decided on that, decided it was needed for some reason. Too large and it may bind. Too small, and it may be sloppy. The end result may be excessive wear and/or unreliable function. When it was time for us to select cam pins, we compared many - and the Colt ones were in spec. The others - not so much. Some were not the right hardness. Some where over-sized. Some were missing the required dry-film lube. The dry film lube is a whole -nother ball of wax because it has its own tolerance for application. So there is tolerance stack, which makes it harder to get into the +- 0.00025 range. Our solution was to make them to a drawing rather than buy them by name from an AR parts vendor. This means, we can check them to the drawing and reject the ones outside the spec. If a company were to just order 'cam pins' then they would have a hard time rejecting any outside the +- 0.00025 range because they have no drawing they can point to as to why they are rejecting it. We also then did not use regular dry-film lube but hardened electroless nickel. This coating has the lubricity but does not wear as much, and can be applied very evenly, leading to fewer parts to reject. So yes, we stick to the mil spec in general, but will deviate when improvements can be made - such as this coating (or for example nitrided barrels rather than chrome lined).

    So... are your cam pins in spec dimensionally? There are plenty out there which are not.
    I think you are drowning in minutia. It matters, but there's another part to my question
    what are the consequences of such an out of tolerance bolt within that spectrum of measurement?
    If, as you say, nobody is dimensionally checking their bolts, and if, as you imply, a dimensionally incorrect bolt is a problem, we would be hearing about mass failures due to.. well due to whatever the specific failure is that this would induce. But we don't.

    What we do hear about, all the time but less so recently, is bolts cracking. Miraculously as it turns out, when any one brand has had an epidemic of cracking bolts it turns out to be a company that is not HPT/MPI. Do all companies that skip HPT/MPI have massive failures? No, but to this point every company I've ever heard of with massive failures has been one that skipped the process.

  6. #96
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    Quote Originally Posted by rsilvers View Post
    Or an incorrect radius tool may have been used.
    This can be troublesome when a shop is trying to stretch out their tooling to last a bit longer.

    But to what you said earlier, is there any good data on parts failures? Amount of hammers that fail, pins that fail, etc?

    I have no problem accepting that the HPT/MPI might not be necessary if Colt/BCM/Daniel Defense/TDP Maker X tells me that they have never seen a bolt fail after the HPT, but I would certainly still pursue parts that were HPT/MPI. I was told a little bit ago that there was not a 100% pass rate after HPT, hence my stance on the issue.

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    Quote Originally Posted by Dirtyboy333 View Post
    Regardless, I would still want MPI bc if it were stopped then there wouldn't be any baseline test and a company might start trying to get by with lower grade or scrap material that wouldn't "pass every time" because their no longer required to.

    To me, MPI is also a test that keeps a manufacturer honest.
    If they were dishonest, they could not tell the truth about doing MPI to begin with. Or they could pass bolts that a govt inspector would fail. If you want to check a maker, have their bolt analyzed at a lab for the type of steel. See if it is C158. The test should be about $50.

  8. #98
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    Quote Originally Posted by 12gauge View Post
    As far as it's usefulness for flaw detection in AR bolts...I think it is very relevant for a couple reasons.

    First, the design of the AR bolt creates sharp corners. These sharp corners are known as stress risers. These stress risers are more or less the area when stressed that the failure will propagate from.(ie the cam bolt hole and lugs)
    Which, interestingly, is the entire point of the KAC E3 bolt, to eliminate those stress risers.


  9. #99
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    Quote Originally Posted by rob_s View Post
    What we do hear about, all the time but less so recently, is bolts cracking. Miraculously as it turns out, when any one brand has had an epidemic of cracking bolts it turns out to be a company that is not HPT/MPI. Do all companies that skip HPT/MPI have massive failures? No, but to this point every company I've ever heard of with massive failures has been one that skipped the process.
    I do think that skipping the HPT/MPI also comes with skipping the correct material or other steps along the way, so it is tough to 100% pinpoint the real problem. All we know is that the right material and right tests yield really good bolts.

  10. #100
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    Quote Originally Posted by orionz06 View Post
    Think of it this way, if you build houses for a living, a tree house for your kids should not be an issue. Are you gonna trust that you are that good and let them climb up into it, or are you gonna head up and jump a few times first?
    This is why we fire a proof round and then test fire every upper. If it is going to blow up, it will probably do it then. And then we check headspace. If lugs moved, the headspace would grow.

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