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Thread: Some notes on HPT/MPI

  1. #1
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    Some notes on HPT/MPI

    Mil-C-70599A is the MILITARY SPECIFICATION for the CARBINE, 5.56MM: M4. This document is readily available online dated 20 January 1994. It is exactly what the name implies. Relevant sections for this discussion are:

    3.4.4 High Pressure resistance. Each barrel assembly and
    bolt shall withstand the firing of one Government standard M197,
    5.56mm high pressure test cartridge conforming to MIL-C-46936.
    After proof firing, parts shall be free of cracks, seams and
    other injurious defects as evidenced by visual and magnetic
    particle inspection
    .
    4.7.4. Barrel inspection. The barrel assembly shall be
    magnetic particle inspected in accordance with MIL-STD-1949
    utilizing a current of 400 to 500 amperes for circular continuous
    magnetization. The barrel assembly shall be examined for
    evidence of cracks, seams and other injurious defects.
    4.7.4.3 Bolt inspection. The bolt shall be magnetic
    particle inspected
    in accordance with MIL-STD-1949 utilizing
    standard five turn magnetizing coil with a current of 200 to 300
    amperes. Both circular and longitudinal continuous magnetization
    with wet fluorescent solution shall be used. The bolts shall be
    examined for evidence of cracks, seams and other injurious
    defects.
    4.7.4 High-pressure resistance test. This test shall be
    performed during individual carbine testing in accordance with
    TABLE III, using a fixture for holding the bolt and barrel
    assembly per drawing 11837944.
    36
    MIL-C-70599A(AR)
    4.7.4.1 Test cartridge. One (1) high-pressure test
    cartridge (see 3.4.4) shall be fired in each bolt and barrel
    assembly. Unless otherwise specified, the bolt and barrel
    assembly shall be tested concurrently. After proof firing
    cartridge cases shall be examined for bulges, splits, rings and
    other defects caused by defective chambers of the barrel
    assembly.
    The document makes reference to the following other military documents relative to the above

    MIL-C-46936 Cartridge, 5.56mm, Test, High Pressure, M197
    MIL-STD-1949 Inspection, Magnetic Particle (this is a 27 page document, also available online)
    INSPECTION EQUIPMENT DRAWINGS 11837944 - Barrel and Bolt Proof Testing Fixture


    There seems to be a lot of discussion of the minutia of the above in various locations recently. In order for one to have an intelligent discussion on the topics, knowledge and understanding of the above documents is necessary. Additionally, it is very easy for a manufacturer to say "we HPT" and/or "we MPI", when in fact what we as consumers should be looking for is "we HPT in accordance with MIL-C-46936" and "we MPI in accordance with MIL-STD-1949".

    This by no means excludes other means or methods of quality control. However, just like a builder must demonstrate how their way is better than the building code if it does not meet it, manufacturers should be expected to demonstrate how their method of quality assurance exceeds the requirements of the relevant documents above.

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    Note, the initial document is dated 20 Jan 1994. I also have a copy dated 31 Jan of the same year. It is entirely possible (and even likely) that there are later documents, but since the reality is that most manufacturers aren't even meeting the 16-year-old spec it is doubtful that they are meeting a newer spec.

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    Thanks for the info Rob!

    Do not taunt Happy Fun Ball...

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    TM43-0001-27 has info on the M197 on page 10-9.

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    Thank you rob s. I have read through lots of your articles and I appreciate everything that you do. Seriously, thanks.


    OK, so unless an manufactuere/vendor specifically states that their barrel or bolt has been "HPT in accordance with MIL-C-46936" and/or "we MPI in accordance with MIL-STD-1949," we can probably assume it wasn't. Correct?



    Bolt-related questions (I apologize in advance if these are stupid questions):

    A reputable manufactuer of high quality non-military AR-15 match barrels offers a 'mil-spec' matched/headspaced bolt as an option with the purchase of a barrel. Without the inclusion of the applicable MIL-STD in their verbal or printed description, is it assumed that the bolt was not subjected to or does not meet the individual HTP or MPI standards? I'll guess yes. So is this an issue when someone goes to build an SPR or DSM-R?

    I have read that individually HPT and MPI test bolts and carriers should be marked as such, yet there is no mention of this in any of the military standards. Is this BS? I recently obtained a Daniel Defense auto BCG, and unless my eyes completely failed me, there were no markings on the carrier or the bolt indicating it was subjected to HPT or MPT testing, despite the manufacturer literature stating so.



    Thanks again rob s.

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    Quote Originally Posted by hikeeba View Post
    I have read that individually HPT and MPI test bolts and carriers should be marked as such, yet there is no mention of this in any of the military standards.
    First, I do not believe that there is any reason to HPT/MPI a carrier.

    Second, Mil-C-70599A DOES call for marking of bolts and barrels that have passed the tests

    4.7.4.4 Proof Markings. Proof marks and magnetic particle
    inspection marks shall be applied on barrel assemblies and bolts that have passed this test.

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    Let me also reiterate that the point of this thread is to act as a jumping off point for discussion. I am NOT saying that any bolt or barrel that hasn't been HPT/MPI is immediately crap. What I am trying to do is make sure that those tossing those terms around are clear with what they mean according to the military specifications as a starting point for the conversation. If, for example, someone can demonstrate that cryogenically treating a bolt and then x-raying for flaws is a better process then that should be considered.

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    Quote Originally Posted by 5pins View Post
    TM43-0001-27 has info on the M197 on page 10-9.
    I'm glad you posted that, as this is where I was headed next.

    M193 Chamber pressure...............52,000 psi
    M196 Chamber pressure ..............52,000 psi (tracer)
    M197 Chamber pressure...........70,000 psi (high pressure test)
    M855 Chamber pressure ..............55,000 psi
    M856 Chamber pressure ..............55,000 psi (tracer)
    M995 Chamber pressure...............50,250 psi (armor piercing)

    Therefore the High Pressure Test M197 round is 33% higher pressure than (or 135% of) M913, and 27% higher than (or 127% of) M855.

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    Quote Originally Posted by rob_s View Post
    Second, Mil-C-70599A DOES call for marking of bolts and barrels that have passed the tests
    Sorry. I must have misremembered what I read. That is also stated word for word in MIL-R-63997B (AR) for the M16A2.



    Just because something is stated as being 'mil-spec,' doesn't necessarily mean it adheres to the exact military specification for that item. And even though a manufacturer states it's product has been HPT and/or MPT, it doesn't necessarily mean that those proceedures were carried out in accordance with a military standard.

    I understand what the military specifications are and why they are in place. I also feel the term 'mil-spec' is used quite loosely in the consumer market. 'Mil-spec' in that the bolt has been properly manufacturer, tested, but mysteriously not marked? Or 'mil-spec' in that it was made to a standard dimension and will work with 'any mil-spec' carrier?

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    I believe the test is the most important. Markings are meaningless as they can be applied when the test has not been performed. Integrity comes into play both ways.

    The point of this thread is sort of to point out that "all HPT/MPI is not created equal".

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