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Thread: Some notes on HPT/MPI

  1. #31
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    Quote Originally Posted by C4IGrant View Post
    From conversations I have had with various companies, I would say that they failure rate is around 3%.


    C4
    Quote Originally Posted by TehLlama View Post
    I'm curious what round count you would peg the 3% failure rate to occur by...
    Wouldn't the round count be one? As in; One (1) high-pressure test
    cartridge

  2. #32
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    Luke I was going to post the same thing until I read the rest of the post. I *think* what he's trying to ask is, if the HPT isn't done on a batch of 100 bolts, at what round count could the 3 bolts that should fail be expected to fail and how.

  3. #33
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    Ahh... Yes, I see that now

    On one hand I look at HP/MPI as a procedure that finds the parts that are flawed from the get go. But, that HP is the key. If one never suffered an overpressure condition in their rifle I expect that a number of that 3% would likely serve through their normal expected service life without failure.

    Then, you have to figure that a certain number of that 3% would "fail" a MPI inspection at some point but not in a way that was user detectable or catastrophic, and may still serve out their expected service life or at least not be drastically reduced.

    Then, some part of that 3% would fail early, detectably, even catastrophically. Those are the ones I don’t want .

    Yes, it would be interesting to know the details of the 3%. How many are catastrophic failures? How many are detectable after the HP without the MPI? Stuff like that. Even more interesting and I suppose more to the point of what TehLlama was getting at would be to take a significant number that "failed" HP/MPI testing, but only in a way that was detectable with MPI, stick them in guns and see when they failed in an obvious/detectable manner.

  4. #34
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    Quote Originally Posted by TehLlama View Post
    I'm curious what round count you would peg the 3% failure rate to occur by. I'm pretty confident that over 95% of all AR's out there now haven't seen round #1001 yet (all but one of mine included, sadly), and I'm assuming the types of failures that would be highlighted and caught early by HPT/MPI testing would increase drastically as the round count got over 5k, and catastrophic failures (I'm assuming bolt lug or separation at the cam pin hole).
    The discussion on this really should be an academic one, but it's the internet, and we're cheap: fact remains that to see wear related failures on rifles it usually costs at least the original cost of the rifle in ammunition to really expect part failures, which among the population of completed ARs is the exception, not the rule.
    Virtually anybody with the experience to budget their rifle AND the rounds they expect for it to see immediately realize the false economy of cheap barrels and BCG's, and just go exclusively with stuff from the half dozen top end manufacturers.

    What makes me the most curious is how many original sources there are for bolts. CMT as far as I know makes a large amount of them, and they get used by lots of other companies (but emerge in many unique flavors - cheap, MPI tested alone, HPT/MPI).
    IMHO, the thing that kills bolts the most is poor heat treating. This failure usually shows itself at 3K and under and the break always happens at the cam pin hole.

    BM currently holds the record for the most broken bolts in this area.


    As your gun is shot, small cracks form naturally. The bolt continues to work just fine for a long period of time AFTER that the cracks have formed.

    Someone will be along shortly to say; "My XTZ bolt is heat treated and so it is GTG and don't need to stinking HPT/MPI.

    The bolts we see break are usually from companies that don't follow the TDP in any way shape or form. This is why buying from companies that acknowledge the TDP/Govt Std and follow it are always going to have a higher quality level than ones that do not.


    C4

  5. #35
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    I'm still waiting for a company to say screw HPT/MPI we're going to x-ray our products because the HPT reduces bolt life by x percent.

  6. #36
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    Quote Originally Posted by thopkins22 View Post
    I'm still waiting for a company to say screw HPT/MPI we're going to x-ray our products because the HPT reduces bolt life by x percent.
    I have heard companies that don't HPT compare it to "testing bumpers by crashing the car into a brick wall at 75 MPH" or words to that effect.

  7. #37
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    Quote Originally Posted by rob_s View Post
    I have heard companies that don't HPT compare it to "testing bumpers by crashing the car into a brick wall at 75 MPH" or words to that effect.
    KAC's III has posted here something along the lines of he doesn't think it's the best possible way to test something because of it...but they do it because their customers demand it.

  8. #38
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    Quote Originally Posted by C4IGrant View Post
    IMHO, the thing that kills bolts the most is poor heat treating. This failure usually shows itself at 3K and under and the break always happens at the cam pin hole.

    BM currently holds the record for the most broken bolts in this area.


    As your gun is shot, small cracks form naturally. The bolt continues to work just fine for a long period of time AFTER that the cracks have formed.

    Someone will be along shortly to say; "My XTZ bolt is heat treated and so it is GTG and don't need to stinking HPT/MPI.

    The bolts we see break are usually from companies that don't follow the TDP in any way shape or form. This is why buying from companies that acknowledge the TDP/Govt Std and follow it are always going to have a higher quality level than ones that do not.


    C4
    It would not be hard to check the heat treat in a separate step to prove that the operation was done correctly. If a company said they follow the TDP except skip the HPT and replace it by checking the hardness on every bolt, that could be viable. Unfortunately, many of the same companies that don't follow the TDP would likely not have a rigorous enough testing method, leading to the same BM scenario.

    Quote Originally Posted by rob_s View Post
    I have heard companies that don't HPT compare it to "testing bumpers by crashing the car into a brick wall at 75 MPH" or words to that effect.
    Quote Originally Posted by thopkins22 View Post
    KAC's III has posted here something along the lines of he doesn't think it's the best possible way to test something because of it...but they do it because their customers demand it.
    There is a large percentage of people who don't care if there is a better way, they want the TDP. It's proven. Until the better way is incorporated into the TDP, it will likely be seen as less desireable by many. Specifications in any large organization are slow to change, and usually it takes overwhelming evidence. The pace may be slow, but they won't get caught up in a fad that looks great at first and then creates problems five years down the road.

    Even if there is something better, Colt is making a lot of M4s every day. If there was a rash of bolts or barrels failing at low round counts, it would be known.

  9. #39
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    I've seen a shit ton of rounds go through Colt M4s. I've worn the barrel out of one in about 3 weeks. Yet I had never heard of a Colt bolt breaking. I never new broken bolts were a problem until I had to carry a BM.

  10. #40
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    Quote Originally Posted by sniperfrog View Post
    I've seen a shit ton of rounds go through Colt M4s. I've worn the barrel out of one in about 3 weeks. Yet I had never heard of a Colt bolt breaking. I never new broken bolts were a problem until I had to carry a BM.
    I have seen them break, usually at the locking lug beside the extractor as I recall however, they were training guns that had fired many rounds.
    Last edited by Redhat; 04-01-10 at 22:54.

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