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Thread: Free-float rail flex

  1. #11
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    One of the many reasons I like MRPs...its not one single thing like a FF rail...a barrel....an upper receiver that cause the ultimate flex in an upper, but rather the sum of all three being separate pieces and the way all three are mated together. This leaves the door wide open to flex, and requires very sound designed components and the methods used to mate them up in order to minimize as much flex as possible.
    Last edited by ALCOAR; 02-07-11 at 17:59.

  2. #12
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    If I were to check for barrel and rail flex I would clamp the complete upper in a vise with the barrel horizontal (using the proper vise clamp).
    The next step would be to establish dial indicators on 3 points of the upper.

    1- on the receiver, flat top before the barrel nut.
    2- forward end of rail on highest point.
    3- Barrel muzzle end on highest point as well.

    Set all indicators to "0" and make sure upper is secure in vise.

    Use a small bottle jack or make up a jack screw tool to be able to move and hold a position. Place this under the rail (below the #2 dial indicator) and apply pressure to move the rail and indicator a given amount, let's say .020" (about 1/2mm). Now log the data on indicator #1 (should still be "0") and #3. Apply additional pressure under rail and log again.

    After a few set points remove pressure and log as well until you are back to "0" on the #2 indicator and now you have all the data to determine how much the rail movement effects the barrel muzzle.

    Remember the #1 indicator should always maintain "0" to show the upper receiver is not moving as a unit. Also you will want to protect the finish of the rail where you apply pressure so make sure whatever you use doesn't deform and skew your data.
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  3. #13
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    Quote Originally Posted by TRIDENT82 View Post
    One of the many reasons I like MRPs...its not one single thing like a FF rail...a barrel....an upper receiver that cause the ultimate flex in an upper, but rather the sum of all three being separate pieces and the way all three are mated together. This leaves the door wide open to flex, and requires very sound designed components and the methods used to mate them up in order to minimize as much flex as possible.
    Is this flex bad, if it does exist? If so why? What are the negative effects?

  4. #14
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    The effects of flex in terms of practical accuracy is very minimal. Most properly free floated rails that mate soundly with both barrel and upper receiver should not experience any discernible negative effects due to the amount of flex being experienced.

    A big reason why I am not an AK fan is due to the overall effect that flex has on the platform due to the design of the handguard, barrel, and receiver.
    http://www.youtube.com/v/L_E_GJayano

  5. #15
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    Interesting vid of the AK flexing.

    Do you have a vid of a AR in slow motion and does AR have some flex?

    When I got my retired sniper pre 64 Mod. 70 rebuilt and glass bedded, my gunsmith told me how even bolt rifles exhibit a degree of barrel flex when shot no matter how thick of a barrel contour.

    Last edited by seabass; 02-07-11 at 20:32.
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  6. #16
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    Barrel Flex

    Looking at the video, it looks like that rail compresses with your finger as you pull on the barrel. All barrels flex, when you sling up tight, and when you just fire it. A camera with a fast enough frame rate can detect that flex when shooting. As long as it returns to origin and it isn't loose then it should be fine.

  7. #17
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    I have 5 carbines with barrels from 10.5" to 18" all with Daniel Defense Lite rails from 10" to 14" and I think the flex is the barrel moving inside the rails, because the heavy barrels flex a lot less than the lighter profiles in the same rails.

    My 16" is very heavy under the hand guards with a steep taper vs. a 14.5" BCM standard barrel both in a 12" lite rail. The standard profile barrel is very easy to flex while the Recce profile is much more difficult.

    My 10.5" LMT which has a very heavy profile barely moves inside the same DD Lite rail.

    Cameron

  8. #18
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    Quote Originally Posted by rob_s View Post
    Is this barrel flex or rail flex? Does it matter? How do you tell the difference?

    How would you test the flex to see if it matters??

    I'm kinda confused & what we are looking for?? Rail Weakness or POI / accuracy??

    When benchrest first started, all the shooters would rest the actual bbl on the sandbag...that is taboo now.

    Are we concerned about the strength of the rail or the impact of the projectile??
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  9. #19
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    I checked both of my free float uppers. One with a 16" military contour bbl / 12" DD omega rail, the other with a 14'5 M4 bbl / 13" Troy TRX rail. Both have very slight flex, and appear to me mostly the barrel doing the flexing. I did the test like your video Robb, but then laid the gun on the floor to support the rail and tested again only to find that the flex was still there. So, my conclusion, its more the barrel than the rail.
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  10. #20
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    From my point of view the only way to properly test rail flex is to find a way to secure the rail to an immovable object. Maybe using a cheap optics mount, mounted to the rail with the optics mount clamped in a vice. Use the position of the optics mount to simulate where on the rail forces on the rail would be input to the rail. IE slings, foregrips etc.

    Once that is setup you apply force to the reciever and measure deflection at the barrel where it exits the rail. Doing it this way takes barrel flex out of the equation and allows us to determine how much the rail flexes in relation to the reciever without flexing the barrel in the process.

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