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Thread: DPMS accuracy

  1. #21
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    Quote Originally Posted by ckmark View Post
    Chrome lining does not improve accuracy in anyway. It is only there to help protect the barrel. For the most part it would actually decrease your accuracy.

    DPMS uses quality barrels. One of the few things they do use that are quality. If you take care of it, it won't break. Seems like their newer BCG's are looking a lot better then the older ones.

    MPI/Shot peening is useless in my opinion, but others will tell you different on here.

    What is the definition of "quality barrel?"

    I am also not aware of how looks have anything to due with longevity/reliability (in regards to BCG quality).

    HPT/MPI is far from useless and IS the current .gov standard.


    C4

  2. #22
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    Quote Originally Posted by .45fmjoe View Post
    What?
    Quote Originally Posted by C4IGrant View Post
    What is the definition of "quality barrel?"

    I am also not aware of how looks have anything to due with longevity/reliability (in regards to BCG quality).

    HPT/MPI is far from useless and IS the current .gov standard.


    C4
    Looks? Their staking has been better. The last few I've seen didn't look like an idiot took a nice wack at a sharp solid object and called the indent it made staking.

    A quality barrel would be one you don't hear people having much trouble with.

    As for the HPT/MPI it is not useful in hardly anyway. Shot peening you could argue to be, but scientifically it is not. I've never seen physical proof that either are better. If you have it I would love to read it, and would greatly appreciate it! It would make me feel a bit better about the money I've spent on my parts.

    Magnetic particle testing only find defects on the surface. It does so because steel is ferromagnetic and the "lines" of magnetization applied in the MT process will concentrate at ALL discontinunities on the SURFACE. This includes all holes, lug corners and any other edges. Such indications are NORMAL. MT's only value here is finding defects on the relatively flat surfaces like the cam pin hole web.

    It cannot find defects located deep in the material. And that includes defects in the center of the cam pin hole web, a HIGHLY stressed area. Defects in other areas outside of the lugs and bolt face.

    Now, specifications for the military are "written in blood". The MT specification (MT is the proper NDT term, Magnaflux is a trademark process and MPI is military) arose out of the early life of the M16, material science has progressed to make such testing unnecessary IF the proper material is utilized. But if the letter of the TDI is followed, lesser materials and specifically poor processing WILL make life short for any bolt.

    ASNT is the most recognized authority in such matters. My experience dealing with ASNT level IIIs with regards to MT and UT is the basis of my judgement and I have had them run MT, PT, RT and UT on some bolts. Their judgements were PT is by far the best NDT for both bolts and carriers due to geometry.

    Now for the materials engineering. Bolts with significant defects that are missed by NDT will fail in a very low cycle fatigue regime. That is 1-100 rounds. The failed part's fracture line will show the defect, usually just below the surface along with large areas of ductile faliure.

    Bolts which fail in the 101-5000 round counts which fail do not do so from any INDICATABLE defect but from MATERIAL FATIGUE. These failures are noted by the absence of any ductile failure. This is typical low cycle fatigue in the hardened steels used in firearms.

    The most common fracture point is the web of the cam pin hole. The most common problem is severity of quench. This is not anything that can be detected by MT/MPI. Buy quality, not specifications.

    Now for the other section of materials engineering. Shot peening. This WILL help by turning the surfaces peened into a material having residual compressive stress. This MUST happen before the heat treatment but the process of heat treatment also involves carburizing the surface of the steel. When the steel is above the austentic point, its structure changes from face centered cubic to body centered cubic, slightly increasing the volume which "opens up" the surface to absorb carbon. When this happens, a slight increase in packing factor which when the metal cools, negates the compressive stress imparted by shot peening. The net effect? Nil.
    http://www.ar15.com/forums/topic.htm...f=118&t=413742
    Last edited by ckmark; 06-05-09 at 12:57.

  3. #23
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    CkMark - while I tend to agree w/you on some points about MPI/HPT and I could careless if the Gov't requests it, quoting someone else on another board (unless that is yourself you are quoting?) is hardly first hand knowledge.

    Have you had the time to verify those statements you quoted?

  4. #24
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    This link is in the thread on ar15.com

    http://www.asnt.org/ndt/primer1.htm

  5. #25
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    Quote Originally Posted by ckmark View Post
    Looks? Their staking has been better. The last few I've seen didn't look like an idiot took a nice wack at a sharp solid object and called the indent it made staking.
    Better? Out of the handful you have seen? How bout they do it perfect EVERY TIME!

    A quality barrel would be one you don't hear people having much trouble with.
    Really? What is the definition of "trouble?" If it is sitting in the safe not bothering anyone would that be classified as not causing any trouble???? Of those people that do not have any "trouble", how many of them actually use their AR??

    As for the HPT/MPI it is not useful in hardly anyway.
    How would you know? Are you a manufacturer that has captured the data on how many bolts have failed MP (after being HPT'd)??? In case you were not aware, it is the current .Mil standard and until something better comes along, it is how bolts are to be done.

    Shot peening you could argue to be, but scientifically it is not. I've never seen physical proof that either are better.
    You have never seen it do any harm either, but just decided that it was not needed???? There is a purpose for shot peening and is called out in the TDP.

    Please do not link info from BARFCOM. Argue YOUR OWN FIRST HAND INFORMATION about what you are talking about. Reciting what someone else said is of little use to us on this forum.


    C4
    Last edited by C4IGrant; 06-05-09 at 13:08.

  6. #26
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    You have no information to prove it otherwise. It would take a fairly decent chunk of change to gather scientific information on this subject anyways.

    I've seen plenty of manufacturers say that MPI/HPT is not useful.

    Just because its military standard doesn't make it good, they have plenty of ridiculous standards.

    This is offtopic anyway. He asked about the barrel anyway, I stated my opinion. Everyone has their own.

  7. #27
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    Quote Originally Posted by ckmark View Post
    You have no information to prove it otherwise. It would take a fairly decent chunk of change to gather scientific information on this subject anyways.
    Yes I actually do. I know why shot peening is called out in the TDP (for instance).

    I've seen plenty of manufacturers say that MPI/HPT is not useful.
    So have I. These are companies that either do NOT understand the TDP or do not want the added expense of going through the process.

    I would suggest that you stop talking to companies that know nothing about the TDP and talk to ones that do (and have .Mil contracts).

    Just because its military standard doesn't make it good, they have plenty of ridiculous standards.
    This is where you are showing your lack of knowledge. The .Mil standard is NOT the highest quality there is. Quite the opposite. Having been in the Military and then worked procurement/awarding contracts, I can assure you that the Military does what it takes to just get by (read lowest quality that will still function).
    So if what the TDP (.Mil spec) calls out is the ENTRY/BASIC quality standard that is acceptable, then what does that make stuff produced by DPMS, RRA, BM, Oly, etc that do NOT come anywhere near the .Mil Spec?????????
    Just to make it clear for you, I DO NOT view the TDP as the end all be all standard. Unfortunately, only 1 or 2 companies are even trying to improve upon the TDP. So until companies start producing products that are ABOVE what the TDP calls out, the TDP will remain as the STANDARD!

    You are entitled to your opinion, but please try and back it up with facts that are based on FIRST hand knowledge. We would also hope that you have had more than one or two experiences with something before trying to pass it off as fact (especially on this forum).


    C4
    Last edited by C4IGrant; 06-05-09 at 13:27.

  8. #28
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    ckmark -

    I would stand down if I were you

  9. #29
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    Interesting how this discussion has mutated from its original intent and devolved, again.
    Last edited by HelloMcFly; 06-05-09 at 17:01. Reason: tasteless quote removed by user

  10. #30
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    Quote Originally Posted by HelloMcFly View Post
    Interesting how this discussion has mutated from its original intent and devolved....once again, please refer to the quote below. Repeat as often as necessary.

    I am sure people in this industry who have special needs children appreciate the reference.

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