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Thread: Free-float rail flex

  1. #21
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    Pretty much the same experience with most here regarding what appears to be the barrel being "flexed" rather than the rail. This is at least what it seems to be doing on my Noveske 16" Light Recce Lo-Pro with them 11" or so SWS rails. Mine seem to "flex" up and down and side-to-side some bit, but not too much.

  2. #22
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    Quote Originally Posted by seabass View Post
    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.

    I have not seen a slo-mo vid of an AR exhibiting flex.

    I would agree w. your gunsmith and in my first post if you note I said.."in order to minimize as much flex as possible". Even the MRPs with a 100% 1piece chassis and a beautifully designed barrel capturing aspect that incorporates a proprietary barrel extension will flex to a minor degree.

    Good way to think about this is:

    Take a barrel by itself and try and bend it w. both hands, not gonna happen.

    Now take a barreled upper receiver with a FF rail and take one hand on the barrel and the other on the rail or the upper receiver and you are gonna detect at least some minor play or movement.

    The reason why movement was present in the second scenario I'm sure is obvious to most...its the mating of the barrel to the upper receiver/FF rail. Some accuse me of being a bit friendly with MRPs but this topic really makes the system shine in regards to rigidity and flex.

    Nice rifle by the way

  3. #23
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    I believe this could be tested with a laser bore sight ,the kind that goes in the chamber. Assuming that the device sends the beam down the center of the bore, if the barrel is flexing more than 1/2 the diameter of the bore the laser should disappear or it will be wildly distorted by reflecting off the inside of the barrel. Might be worth a try.

    You could also try to flex the barrel up in the vertical plane (relative to the receiver). If the barrel is flexing, the gas tube should move deeper into the receiver when this is done.

    Vince
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


  4. #24
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    My DD rails don't flex really, my TRX-E rails flex a ton...

  5. #25
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    Quote Originally Posted by TRIDENT82 View Post
    Take a barrel by itself and try and bend it w. both hands, not gonna happen.

    Now take a barreled upper receiver with a FF rail and take one hand on the barrel and the other on the rail or the upper receiver and you are gonna detect at least some minor play or movement.

    The reason why movement was present in the second scenario I'm sure is obvious to most...its the mating of the barrel to the upper receiver/FF rail.
    I don't agree with your analysis of the situation.

    Some accuse me of being a bit friendly with MRPs but this topic really makes the system shine in regards to rigidity and flex.
    We get it.
    But to what advantage? Didn't you yourself say that flex doesn't matter?

  6. #26
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    On a more general note...

    I think that what we really have is at least 4 different potential issues, and I'm going to define them this way
    1. Barrel flex. The condition whereby the barrel moves from it's centered location, under an external force, and returns to the centered location.
    2. Forearm flex. The condition whereby the forearm moves from it's centered location, under an external force, and returns to the centered location.
    3. Barrel displacement. The condition whereby the barrel moves from it's centered location and does not return to the same centered location.
    4. Forearm displacement. The condition whereby the forearm moves from it's centered location and does not return to the same centered location.


    I further submit that # and #2 not only don't matter, but are also unavoidable. Even if you think they aren't happening, they are. #1 and #2 may be an issue if the barrel is being flexed at the moment of the shot, or if the forearm is being flexed and the front iron sight is being used and is attached to the top rail of the forearm. Still, these are only temporary problems as once the external force is removed the flexing part will return to it's centered state.

    #3 and #4 are the problems. If parts are moving out of alignment and not coming back to the same position, or very nearly the same position, every time, then accuracy (even gross accuracy) cannot be repeatable. If a barrel nut is allowing the barrel to be displaced in the upper, or a forearm mounting system allows the forearm to be displaced about the barrel, you may have issues. But even then, forearm deflection is only an issue if iron sights, attached to the forearm, are being used. Repeated displacement may be disconcerting, and instill a lack of confidence in one's equipment, and may even be indicative of a situation that could lead to part failure or separation (barrel or forearm coming loose from the gun, depending on where the loose part is), but in and of itself it is a non-issue.

    Caveat here. I'm not talking about gnat's-ass, benchrest, sub-MOA shooting. Not only do I have no interest in same, I'm not qualified to talk about it due largely to that lack of interest leading to a lack of pursuit. I'm talking about chrome-lined, Govt, lightweight, or M4 profile, carbon steel, button-rifled or hammer forged, standard AR barrels shooting the kinds of ammo that most of us find ourselves shooting most often such as Wolf, XM193, Prvi, PMC, Sellier & Bellot, etc. in the kinds of applications the guns are intended for.

  7. #27
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    for some that have not seen barrel flex in an ar.

    this a a promo vid for LAV carbine vid,

    1:37 sec shows some barrel flex/whip etc in a 16' light weight profile carbine.

    http://www.youtube.com/watch?v=5hKaPkvOtEo

  8. #28
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    Pressing the barrel as done in Rob's video will cause both the barrel and the forearm to flex. In this case, the barrel should show more flex than the forearm.

    The reason a barrel will flex while mounted in a receiver and will not when attempting to bend an unmounted barrel with just your hands is a matter of leverage and deflection. Greater leverage is available when flexing the mounted barrel. Also, the mounted barrel has a longer length from it's fulcrum and the deflection will show more displacement with the greater distance.

    The main purpose of free floating the barrel is to eliminate any interference with the natural flexing of the barrel during the shot. On an AR, there will be some from the gas tube, but that should be minimal. Secondary is to eliminate flexing the barrel and changing the point of impact through pressure from a sling, vertical grip or simply from being gripped tightly by an adrenalin pumped shooter.

    Pressure on a free float forearm will be transferred to the upper receiver and possibly cause the upper to flex. How much? Here are two links to actual tests performed by K.L. Davis
    http://m4carbine.net/showthread.php?t=57
    https://www.m4carbine.net/showthread.php?t=58

    I don't offer these tests as a final conclusion but as facts for further discussion
    Last edited by MistWolf; 02-28-11 at 15:14.

  9. #29
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    Like MistWolf said, the amount of static force on the hand guard or bbl (like from using a sling or VFG) can be substantial. Distance from handguard attatchment point/bbl nut to pressure point multiplied by force, even 14 inches down putting 20 lbs of pressure is like putting 280lbs of weight on it.

    You would probably bend the upper/strip threads or break the forearm before you permanently (plastic-ly) bent the barrel though. The links above do a good job of showing that.
    Last edited by ZRH; 02-28-11 at 18:06.

  10. #30
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    Actually, 20 lbs of force with 10 inches of arm is 200 in/lbs of force. That's 16.7 ft/lbs
    Last edited by MistWolf; 02-28-11 at 18:10.

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