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Thread: Vltor gas block/front sight assy torque test

  1. #11
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    Quote Originally Posted by KevinB View Post
    Interesting...

    I am more curious about frontal impacts howevever -- since I have seen two PRI's come out of alignment from that.
    Perhaps this is because most people use these on free-float barrels, which do not use the regular handguard cap. This leaves a gap between the front of the seating shoulder and the gear of the gas block; this gap is a potential failure point, as it gives space for an impact to move the gas block back.

    I'm having an upper built with a PRI gas block, and my gunsmith is putting a shim behind the PRI to fill the gap.

  2. #12
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    Quote Originally Posted by wild_wild_wes View Post
    Perhaps this is because most people use these on free-float barrels, which do not use the regular handguard cap. This leaves a gap between the front of the seating shoulder and the gear of the gas block; this gap is a potential failure point, as it gives space for an impact to move the gas block back.

    I'm having an upper built with a PRI gas block, and my gunsmith is putting a shim behind the PRI to fill the gap.
    True -- as for MM's comment we also had a PRI shift at the Benning Sniper comp one year - the single shot Cdn's suppressed SPR gun...
    After that one I had barrels for FF's profiled not to need the gap for the handguard cap, and pinned...
    I know that the sights where not locked up when the impacts took place.

    I'm also bringing .mil gun issues into this -- but any AR type gun I get I ensure it will be able to do duty work too.
    Kevin S. Boland
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    FN America, LLC
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  3. #13
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    i actually brought up shims/washers to eric when the test was done. first, i asked why not make the vltor gas block a bit longer so it butts up against the step, and eric said that (as kevin had done) some barrel manufacturers are making barrels with the step moved forward so there's no gap, so the gas block is automatically centered above the gas port and also prevents movement of the gas block rearward if struck. if he made the vltor block longer, it wouldn't be compatible with those, so a shim is the correct solution. if they hear that it's needed with theirs, they'll look into it. i just think it should be supplied anyways so the gas block hole is centered on the port automatically.

  4. #14
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    Years ago I had a (I beleive his first) 11.5 Colt barrel done with a GG&G flip front via Wes Grant -- he used a shim to position it for the pinning - and then the shim was removed.

    For 99% of users I am sure the crossbolt method works -- I'm just a paranoid.
    Kevin S. Boland
    Manager, Federal Sales
    FN America, LLC
    Office: 703.288.3500 x181 | Mobile: 407-451-4544 | Fax: 703.288.4505
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  5. #15
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    Quote Originally Posted by militarymoron View Post
    i actually brought up shims/washers to eric when the test was done. first, i asked why not make the vltor gas block a bit longer so it butts up against the step, and eric said that (as kevin had done) some barrel manufacturers are making barrels with the step moved forward so there's no gap, so the gas block is automatically centered above the gas port and also prevents movement of the gas block rearward if struck. if he made the vltor block longer, it wouldn't be compatible with those, so a shim is the correct solution. if they hear that it's needed with theirs, they'll look into it. i just think it should be supplied anyways so the gas block hole is centered on the port automatically.
    I bought a box of 0.032" thick 0.75" ID shims.
    I use these to account for the missing handguard cap when installing gas blocks.

    When pinning, I also remove the shim like was mentioned in the post above.
    Note the PRI picture I posted above also shows the gap from the shim...
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  6. #16
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    Why do you remove the shim, Randall?

    By the way, I'm having the PRI put on a Sabre 18" barrel. My gunsmith said the usual gap runs about .030", but mine is nearly at .060"....so there is some variability on this, depending on manufacturer.

  7. #17
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    Quote Originally Posted by wild_wild_wes View Post
    My gunsmith said the usual gap runs about .030", but mine is nearly at .060"....so there is some variability on this, depending on manufacturer.
    A few years ago I put a PRI flip up (clamp style) on a DPMS M4 cut barrel and it was .058" (IIRC). I think there was some variables at work here. Possibly the handguard cap edge was too far back on the barrel (I didn't measure it to the barrel extension), or the PRI gas port was machined too far forward in the gas block and/or the gas port was too far forward on the barrel. Or it was a combination of the 'stacking' tolerances everything being on the edges of the allowable tolerances.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  8. #18
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    Quote Originally Posted by wild_wild_wes View Post
    Why do you remove the shim, Randall?

    By the way, I'm having the PRI put on a Sabre 18" barrel. My gunsmith said the usual gap runs about .030", but mine is nearly at .060"....so there is some variability on this, depending on manufacturer.
    I do the measurements and shim as required to center the ports.
    I remove the shim(s) after pinning because it's ugly to have a stainless shim on a black barrel.
    On stainless barrels, I would leave them in as they don't stick out like a sore thumb.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  9. #19
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    Any chance Vltor could be convinced to perform a follow up trial and test the clamp on style gas block for impact resistance? I realize an in depth analysis setup would not be possible, but they could smack the gas block into a few solid objects or hit the gas block with a hammer(afterall it is already wrecked) to get a subjective evaluation of its resistance to impact.

    After reading the test, and the comparison to the motorcycle triple tree application, I am convinced the Vltor clamp style gas block (if properly assembled, loctited and staked) would be able to take a lot of abuse. Motorcycle triple trees are subjected to a lot of forces constantly. I can't think of the last time I have seen a triple tree come loose (dirtbikes and streetbikes). I know I have trusted the setup at 170+ MPH. Not an exact apples to apples comparison, but the fundamental design concept is the same. Anything mechanical can always fail, but this test and topic has definitely given me some food for thought. I may have to get one just to see if Vltor won't do some subjective impact tests.

  10. #20
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    Quote Originally Posted by rpmwfo View Post
    Any chance Vltor could be convinced to perform a follow up trial and test the clamp on style gas block for impact resistance?
    they might, if you contribute a barrel for the test.

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