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Thread: Bolt fails to stay open on PMAG M3 but ok with Lancer L5 AWM mag

  1. #1
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    Bolt fails to stay open on PMAG M3 but ok with Lancer L5 AWM mag

    I am trying to diagnose a problem with a new upper receiver that doesn’t keep the bolt open after the last round has been fired. The problem always occur with the PMAG M3 and not with the Lancer L5 AWM. Both magazines are 30 rounders.

    The upper consists of the following:
    Vltor MURA
    Young Manufacturing M16 chrome BCG
    Lilja 16” carbine gas length
    Geissele Super gas block (gunsmith pinned)

    Ammo: Lake City XM855

    Symptoms:
    Both PMAG and Lancer magazines with 5 rounds loaded cannot be easily inserted into a close bolt
    Shells are being ejected at the 3’o clock position
    No fail to load or fail to fire incidents

    Observed problems:
    Both PMAG and Lancer magazines with 5 rounds loaded cannot be easily inserted into a close bolt
    PMAG never keeps the bolt open after the last round has been fired
    Lancer the bolt remain open after the last round has been fired

    So what was is the lower's configuration and how does that work? The answer is that it functions superbly.
    Same ammo and magazines.
    Zero symptoms with shell ejection at 4:30 o’clock. Magazines can be inserted into a close bolt – no problems.

    The lower consists of the following:
    Mega Arms billet lower
    Spikes Buffer ST-T2 (4.1oz)
    Sprinco blue buffer spring

    The upper consists of the following:
    Mega Arms billet upper
    WMD nickel boron M16 BCG
    Rainer Arms 16” mid-gas length
    Geissele Super gas block (gunsmith pinned)


    Any suggestions on the next step?
    Last edited by patrick0525; 01-15-14 at 12:16.

  2. #2
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    Two questions:

    1) With the new upper receiver, can an empty PMAG be easily inserted with the bolt forward/closed?
    2) With an empty PMAG inserted, and the bolt forward, pull the charging handle all the way back (and then let go of the charging handle). Does the bolt lock back now?

    Joe Mamma
    "Reliability above all else"
    NRA Certified Pistol and Rifle Instructor, Life Member
    Glock Certified Armorer
    Beretta & Sig Sauer Certified Pistol Armorer
    Colt Certified 1911 & AR-15/M16/M4 Law Enforcement Armorer

  3. #3
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    Quote Originally Posted by Joe Mamma View Post
    Two questions:

    1) With the new upper receiver, can an empty PMAG be easily inserted with the bolt forward/closed?
    2) With an empty PMAG inserted, and the bolt forward, pull the charging handle all the way back (and then let go of the charging handle). Does the bolt lock back now?

    Joe Mamma
    1) Yes, using the new receiver with the bolt forward/closed, the PMAG easily inserts with nice click sound.
    2) Yes, with the PMAG still inserted the charging handle and pulled back all the way back, the bolt locks back behind the bolt catch assembly, and then the charging handle is push forward with no buffer spring tension on it.

    Update (6:48am EST): I tried a slow and a fast as possible charging handle pull back. In both situations, the bolt locks back properly. I also need to redo and determine at what o'clock position brass ejection was occurring. I was at indoor range and my wife was watching. She claims 3:00 o'clock, but the funny thing was that there was no brass to pick-up behind me after 50 rounds. All the brass was in the range lane at 1 to 2 o'clock. When I was using the working benchmark rifle(Mega upper -Rainer Arms 16" mid-gas length), I always had to clean-up and pick-up brass.


    Patrick
    Last edited by patrick0525; 01-15-14 at 06:48.

  4. #4
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    Looks like your bolt catch is working and magazine follower is engaging it properly. I've seen similar problem with Magpul P-Mag gen 2, P-Mag20 and E-mag (I have Gen 3 mags, but I did not test them) on some fast cycling rifles, while Lancer L5 and L5 AWM worked fine. Unscientific finger pressure test makes me feel, that Lancer is using much stronger spring and this allow follower to reach bolt catch and push it up faster and with more force (AFAIK Magpul addressed this concerns in Gen 3 mags, but Lancer springs are still stronger that those on Gen3 - to my "finger scale" at least). Similar issue would be with rifle that has (for some reason, for eg short stroking) reduced BCG travel only very slightly. Other reason could be some slight drag or resistance in bolt catch itself, that can slow down it's activation with weaker mag spring.

    Anyway, for testing purposes, try to fit Lancer mag spring into Magpul body and test. This should show if problem is in this area. However remember that actual problem source can lay in different place - to high BCG velocities on shortened BCG travel or drag on bolt catch (or something completely different).

    Users that had this problem (including military users) solved this by switching to Lancer mags, because they could not "fix" their rifles: on very fast cycling rifles like early type, suppressed, 10.4" HK416, Magpul old style mags had problem with feeding rounds fast enough - those beasts can reach 1200rpm in some conditions.
    Last edited by montrala; 01-15-14 at 05:42.
    Montrala

    I'm sponsored competition shooter representing Heckler&Koch, Kahles, Hornady and Typhoon Defence brands in Poland, so I can be biased

    http://montrala.blogspot.com

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    Quote Originally Posted by montrala View Post
    Looks like your bolt catch is working and magazine follower is engaging it properly. I've seen similar problem with Magpul P-Mag gen 2, P-Mag20 and E-mag (I have Gen 3 mags, but I did not test them) on some fast cycling rifles, while Lancer L5 and L5 AWM worked fine. Unscientific finger pressure test makes me feel, that Lancer is using much stronger spring and this allow follower to reach bolt catch and push it up faster and with more force (AFAIK Magpul addressed this concerns in Gen 3 mags, but Lancer springs are still stronger that those on Gen3 - to my "finger scale" at least). Similar issue would be with rifle that has (for some reason, for eg short stroking) reduced BCG travel only very slightly. Other reason could be some slight drag or resistance in bolt catch itself, that can slow down it's activation with weaker mag spring.

    Anyway, for testing purposes, try to fit Lancer mag spring into Magpul body and test. This should show if problem is in this area. However remember that actual problem source can lay in different place - to high BCG velocities on shortened BCG travel or drag on bolt catch (or something completely different).

    Users that had this problem (including military users) solved this by switching to Lancer mags, because they could not "fix" their rifles: on very fast cycling rifles like early type, suppressed, 10.4" HK416, Magpul old style mags had problem with feeding rounds fast enough - those beasts can reach 1200rpm in some conditions.
    The bolt catch assembly is a Seekins version 2. They were kind enough to take back an older Seekins bolt catch assembly and made a small modification for reliability. The bolt catch assembly arm is sufficiently long enough to engage the magazine follower.
    Last edited by patrick0525; 01-15-14 at 15:45.

  6. #6
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    Quote Originally Posted by patrick0525 View Post
    The bolt catch assembly is a Seekins version 2. They were kind enough to take back an older Seekins bolt catch assembly and made a small modification for reliability. The bolt catch assembly arm is sufficiently long enough to engage the magazine follower.
    Let me ask about the details of this. Immediately after *firing* the last round, when your bolt does not lock back on a PMAG: when you then pull the charging handle back, does (a) the bolt lock back (meaning the follower-bolt catch are touching properly) or (b) the bolt not lock back (meaning the follower has possibly slipped past the bolt catch)?

    Joe Mamma
    "Reliability above all else"
    NRA Certified Pistol and Rifle Instructor, Life Member
    Glock Certified Armorer
    Beretta & Sig Sauer Certified Pistol Armorer
    Colt Certified 1911 & AR-15/M16/M4 Law Enforcement Armorer

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    Quote Originally Posted by Joe Mamma View Post
    Let me ask about the details of this. Immediately after *firing* the last round, when your bolt does not lock back on a PMAG: when you then pull the charging handle back, does (a) the bolt lock back (meaning the follower-bolt catch are touching properly) or (b) the bolt not lock back (meaning the follower has possibly slipped past the bolt catch)?

    Joe Mamma
    This weekend, I have to go to the range and check for either condition a or b. When it happened, i was very annoyed and immediatley pressed the magazine release to drop the PMAG.

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    Quote Originally Posted by Joe Mamma View Post
    Let me ask about the details of this. Immediately after *firing* the last round, when your bolt does not lock back on a PMAG: when you then pull the charging handle back, does (a) the bolt lock back (meaning the follower-bolt catch are touching properly) or (b) the bolt not lock back (meaning the follower has possibly slipped past the bolt catch)?

    Joe Mamma
    Update[01/19/2013 5:15 EST] Just got back from the range. The answer to your question is A: when you then pull the charging handle back, does (a) the bolt lock back (meaning the follower-bolt catch are touching properly) The problem is only with the PMAG M3. The Lancer has no problems the bolt locking back. By the way, I asked someone to watch the brass. It was ejecting at 1:00 to 2:00 o'clock

    Comments /suggestions?

  9. #9
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    Quote Originally Posted by patrick0525 View Post
    By the way, I asked someone to watch the brass. It was ejecting at 1:00 to 2:00 o'clock

    Comments /suggestions?
    Your rifle may be overgassed. That means too fast cycling and follower do not move up fast enough and strong enough to lift bolt catch in time (Lancer mags work good, because of strong springs). Heavier buffer (H2 or H3) or Vltor A5 system can solve your problem.
    Montrala

    I'm sponsored competition shooter representing Heckler&Koch, Kahles, Hornady and Typhoon Defence brands in Poland, so I can be biased

    http://montrala.blogspot.com

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    Quote Originally Posted by montrala View Post
    Your rifle may be overgassed. That means too fast cycling and follower do not move up fast enough and strong enough to lift bolt catch in time (Lancer mags work good, because of strong springs). Heavier buffer (H2 or H3) or Vltor A5 system can solve your problem.
    +1

    I had same overgassed issue on an upper I used to own. It would inconsistently not lock back on Pmags.

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