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Thread: HK MR762A1 Failure to Feed

  1. #21
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    Check the piston rings. With your overly frequent cleaning schedule you may have done more harm than good if you went at the piston with any vigor. If there isn't a good gas seal anymore due to damage to the rings, you could potentially affect bolt velocity enough that it leads to numerous failures like feeding or locking open.

    Remove the rod and such and insert the piston by itself into the gas block, then hold the rifle vertically. If the piston falls out easily then replace the rings.

    Also, have you contacted HK yet? Their CS is very helpful, and I'm not sure if you'd have to pay shipping or not but sending it in for them to look at would probably be more useful than asking for help on an online forum for reliability issues with a generally reliable firearm. If there's something wrong with it that's unique to your rifle then it's unlikely that we could properly diagnose it.

  2. #22
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    I’m not too sure it’s the gun. From the last range trip report it sounds like a bad batch of Federal SMK that lacks enough powder.

    That one batch of Federal lacks the power to fully cycle the bolt, thus the failure to reload and lock bolt back on empty mag. But if works with all the others. Toss that batch and ask dealer/store for refund on bad Ammo.

    If it were your piston it would occur with all Ammo unless you have compressed loads in all your other bullet types creating so much gas expansion force that it is blasting your piston back regardless of the alignment of your piston rings. Although I do advocate for ensuring your rings are off-set every time you clean and shoot.

    Just me layman’s $0.02.
    "Just because you're paranoid doesn’t mean they're not after you!"

  3. #23
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    Quote Originally Posted by RegularGuy461969 View Post
    I’m not too sure it’s the gun. From the last range trip report it sounds like a bad batch of Federal SMK that lacks enough powder.

    That one batch of Federal lacks the power to fully cycle the bolt, thus the failure to reload and lock bolt back on empty mag. But if works with all the others. Toss that batch and ask dealer/store for refund on bad Ammo.

    If it were your piston it would occur with all Ammo unless you have compressed loads in all your other bullet types creating so much gas expansion force that it is blasting your piston back regardless of the alignment of your piston rings. Although I do advocate for ensuring your rings are off-set every time you clean and shoot.

    Just me layman’s $0.02.
    Can you clarify what you mean by "Although I do advocate for ensuring your rings are off-set every time you clean and shoot"? I'm waiting for my MR762 (ordered in January) and would like to know if there's anything I need to watch for regarding the piston rings. Thanks!

    Also, are these the right rings https://www.hkparts.net/shop/pc/HK-P...ombo-13p48.htm ? I see that they're marked for MR562 but wasn't sure if they share the same piston.
    Last edited by douglasjboehme; 02-27-2018 at 03:18 PM.

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  5. #24
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    @douglasjboehme

    Yes those are the correct rings. The same piston is issued on the 416/556/762/417/G28/G36.

    To clarify, you need to ensure that your piston rings are not in allignment. This means that the space in each ring is misaligned with the adjacent one. If all of the spaces are aligned the gas will vent through them thereby reducing the gas pressure on the piston and the ability to pushe the op rod and bolt rearward. The Copper Rings act like O rings. They “seal” the piston into the gas block. If the ring openings are aligned, no seal.

  6. #25
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    Quote Originally Posted by RegularGuy461969 View Post
    @douglasjboehme

    Yes those are the correct rings. The same piston is issued on the 416/556/762/417/G28/G36.

    To clarify, you need to ensure that your piston rings are not in allignment. This means that the space in each ring is misaligned with the adjacent one. If all of the spaces are aligned the gas will vent through them thereby reducing the gas pressure on the piston and the ability to pushe the op rod and bolt rearward. The Copper Rings act like O rings. They “seal” the piston into the gas block. If the ring openings are aligned, no seal.
    Makes perfect sense, thank you. This is what I get for asking questions before I see one first hand. :)

  7. #26
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    The piston rings also wear down and lose their surface treatment through heavy use or through "cleaning" if people use brushes or scrapers in attempts to remove carbon fouling. It's worth at least checking the OP's piston rings with the "does it fall out easily" test, based on the vigorous cleaning regimen that he talked about.

  8. #27
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    @dragnet

    Agree fully on always checking piston and rings.

    But I have the same MEN Ammo OP is shooting and it doesn’t seem hot. And the Federal Gold Match, Norma and Ruag I’ve put through mine was really not any different other than heavier bullets and heavier load to accommodate said heavier bullet.

    @douglasjboehme

    Don’t sweat it, that’s why this place is a great forum. “People helping people, it’s a beautiful thing!”
    Last edited by RegularGuy461969; 02-28-2018 at 12:22 AM.

  9. #28
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    OK...replying to a couple of the suggestions here...thanks for all the suggestions:
    1) Over cleaning the piston rings: I have ~ 300 rounds through the gun, I have only cleaned the piston once and that was after it started misfeeding. It was pretty dirty. I used Hoppes bore cleaner on it, let it soak for a few minutes and then wiped it all off. I then lubricated it with FP-10. A few days later in response to a post, I took the piston out again and wiped the lubricant off it and re-inserted. I did the "see if the piston falls out test" a few minutes ago and even after vigorously shaking the the rifle, it didn't drop out. I removed the piston and checked the rings. They looked good. See attached pictures.
    2) Bolt Lock - I don't think it was clear in my earlier post....none of the Federal SMK rounds achieve bolt lock after the last round. I have tried the SMK 168 from three different batches (different manufacture date) and one box of SMK 175. The MEN 147 does lock back after the last round. Note that only one batch of SMK 168 has the failure to feed (the one from May 2017). All the others feed fine, they just don't lock the bolt open on the last round.
    3) I had noticed this before and wasn't sure if it was related, but the charging handle is showing some wear. Seems odd after ~300 rounds. One edge of if that makes contact with the bolt group has the blueing worn off. It is smooth (and definitely well lubricated). Pictures are attached. HKParts.net has one for $150. I may order it and see if that remedies the feeding issue....BTW, any other suggestions for where to buy HK original parts. The HK website seemed pretty lacking (although had better prices than HKParts.net)...they did not have the charging handle.
    4) I am going to try different match ammo Black Hills and Hornady. I also have some Federal XM80CL 7.62 .... I know, not match, but it is Federal, but 7.62 instead of 308.
    5) I plan to contact HK Customer Support tomorrow

    HK MR762A1 Failure to Feed-piston-1.jpgHK MR762A1 Failure to Feed-piston-2.jpgHK MR762A1 Failure to Feed-charging-handle-1.jpgHK MR762A1 Failure to Feed-charging-handle-2.jpg
    Last edited by hkuser9817; 02-28-2018 at 01:44 AM.

  10. #29
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    I sent my MR762 to HK last week, got it back this Wednesday. They indicated that it was a burr in the gas block. They fixed it and tested with the ammo I had indicated had problems (Federal Gold Medal Match 168 and 175). I got out to the range today and fired around 60 rounds through it of the same ammo that had exhibited the failure to feed. Everything worked great with different magazines nd different number of rounds in the magazines. Also after the last round the bolt stayed lock back as it is supposed to. Very happy to have that HK reliability back!

    In addition, the entire experience with HK customer service and support was outstanding. They provided FedEx shipping to their facility and FedEx-ed it back to me within a week. They kept me informed of progress and were happy to explain what they were doing and what they had found. This is hands down the best experience I have had with any customer service. I only regret not sending it back a couple of months ago and wasting all that time and money on trying different ammo.
    Drooler and THRLSEKR like this.

  11. #30
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    ^^^i'm glad everything worked out well...enjoy your beast.

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