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

  1. #21
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    Quote Originally Posted by patrick0525 View Post
    I am not sure what you asking. But the barrel link is the following: http://www.riflebarrels.com/products...es/M4_navy.PDF
    Thanks for the link. I wanted to know the outside diameter of the barrel at the gas port. On your barrel, the drawing says it's 0.750" which is pretty standard.

    If your barrel was a thick heavy match style barrel (thicker than 0.750" at the gas port), it would make sense (to me) to have a relatively large gas port.

    If my math is right, your port size (the area which is the radius squared times pi) is about 62% bigger than the Colt port. Keep in mind that a lot of smart people think even the Colt port size is too large for a 16" bbl with carbine gas system (but right for a 14.5" bbl with carbine gas system). But also keep in mind that there are a lot of overgased ARs out there that run fine.

    The bottom line is that your bolt/carrier velocity is probably too fast for the PMAG spring.

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  2. #22
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    If its mid length with a blue spring and stt2 heavy buffer throwning at 1:30,It seems like there is a lot of gas ...IE over gassed..That set up should work on carbine length gas system..seems like that setup on a mid length would short stroke some,but if its throwng the emptys forward like that it seems like the bolt is really moving fast..

  3. #23
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    Quote Originally Posted by Joe Mamma View Post
    The bottom line is that your bolt/carrier velocity is probably too fast for the PMAG spring.
    And who wrote that in post #4? Who?

    Seriously, best way to "salvage" this upper seems to be competition style adjustable gas block (I mean model that offers several settings to fine tune gas system like Syrac, not just "normal+suppressed" style). Even if OP will decide to stick to Lancer mags only, this will cure symptom, not cause. Upper still will be overgassed and this can bring host of problems (like increased wear on parts, ejection problems, feeding problems). Adjustable gas block will contain problem at it's source.
    Last edited by montrala; 01-21-14 at 07:11. Reason: spelling, added Syrac example
    Montrala

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  4. #24
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    Quote Originally Posted by texasgunhand View Post
    If its mid length with a blue spring and stt2 heavy buffer throwning at 1:30,It seems like there is a lot of gas ...IE over gassed..That set up should work on carbine length gas system..seems like that setup on a mid length would short stroke some,but if its throwng the emptys forward like that it seems like the bolt is really moving fast..
    To clarify, in the mid-gas length configuration with the rainier barrel, blue spring and stt2, brass ejection is about 4:00 o'clock.

  5. #25
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    Because PMAGs run with less friction, the cyclic rate on F/A guns is generally just a tiny bit higher (or on semi guns, the cycle time is shorter) than with metal magazines. This works out fantastic when dealing with crappy, dirty conditions, but on an an overgassed gun, once the cyclic rate (or equivalent semi-auto cycle time) goes over 1100-1150rpm (which is very high--we can only get there for testing with a suppressed mk18 with a ridiculously large gas port, that only gets that fast when hot) then you can experience the bolt not locking back, as you have.

    It sounds like you're on the hairy edge of working/not working if it works with any other mag and not the PMAG, but from the port size, I concur with general sentiment...that's a big port, and likely very, very fast cycling.
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  6. #26
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    Quote Originally Posted by SPQR476 View Post
    Because PMAGs run with less friction, the cyclic rate on F/A guns is generally just a tiny bit higher (or on semi guns, the cycle time is shorter) than with metal magazines. This works out fantastic when dealing with crappy, dirty conditions, but on an an overgassed gun, once the cyclic rate (or equivalent semi-auto cycle time) goes over 1100-1150rpm (which is very high--we can only get there for testing with a suppressed mk18 with a ridiculously large gas port, that only gets that fast when hot) then you can experience the bolt not locking back, as you have.

    It sounds like you're on the hairy edge of working/not working if it works with any other mag and not the PMAG, but from the port size, I concur with general sentiment...that's a big port, and likely very, very fast cycling.
    I concur being over-gassed. The option is to use an adjustable gas-block, however the Geiselle super gas block doesn't come in a adjustable gas block version. The barrel has been gunsmith drilled/pinned with two set screwed dimples. The obvious choice is to switch to an adjustable version Geiselle super gas block if it becomes available.

  7. #27
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    I can probably use a Seekins Precision adjustable gas block and re-use the set-screw dimple areas underneath the barrel.
    Now a new question. The Geissele center-to-center set screw dimension is 0.400" and the Seekins dimension is 0.460".

    With the intent of using both Seekins set screws, is this going to be problem?

  8. #28
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    Contact Clint@blackrivertactical and ask about his insertable gas port.

    Quote Originally Posted by patrick0525 View Post
    I concur being over-gassed. The option is to use an adjustable gas-block, however the Geiselle super gas block doesn't come in a adjustable gas block version. The barrel has been gunsmith drilled/pinned with two set screwed dimples. The obvious choice is to switch to an adjustable version Geiselle super gas block if it becomes available.



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  9. #29
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    Quote Originally Posted by Iraqgunz View Post
    Contact Clint@blackrivertactical and ask about his insertable gas port.
    Thanks, this forum has the best technical support. The link for the low-profile gas block port insert: http://www.blackrivertactical.com/co...09&id=18000700

  10. #30
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    Hi Patrick,

    A CustomTune Gas Port in either .060" or .063" should make it right.

    This can be put right into your pinned SBG, so you can keep it.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
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    BRT CustomTUNE Gas Ports

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