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Thread: "Truing" an AR's receiver face?

  1. #121
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    Going to sit back and wait for Markm's results.
    I am just curious to all hell on this matter of over cranking iron sights to shoot to point of aim.

    Clarence
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  2. #122
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    Yeah, that is the one reason I keep looking at this thread. I guess one has to keep in mind that the relation of the upper's rail to to barrel/bore needs to be considered.

    A question for Markm; do optics mounted to the same upper require as much windage as the rear iron sight? What, if in the case of something like a T-1, if it was moved to the near the same position as the rear sight, does it require as much windage?

  3. #123
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    Quote Originally Posted by BrigandTwoFour View Post
    Hey guys, had a thought.

    I've seen some people before talk about truing up the front of the receiver extension to make a perfectly level and squared up contact between the barrel extension and the receiver as a method to really tighten up the accuracy of the gun.

    I never thought much about it because I wasn't interested in squeezing every last bit of accuracy out of my guns....yet.

    I've come across a few companies that now do it as a service package for accurizing an AR.

    Do you think there is merit to this process? Would there be any appreciable gain? Or is this all just pixie dust and unicorn jizz?

    The gun I have in mind is a precision .308 project that is currently at a stand still due to the panic, but will ultimately turn into a no-expenses-spared kind of thing (well...not literally...I'm not that rich).
    Unfortunately, truing the face of the barrel socket on a receiver is a fruitless endeavor. There are too many other variables, such as barrels with bores that are not drilled perfectly and exhibit axial alignment anomalies. This actually affects more barrels than you think, and no manufacturer is immune from turning out these types of barrels. Most people who own AR rifles own at least one rifle with such a barrel, but will never know since it is not visible to the naked eye and the rifle will sight in fine. Also, there exists the possibility of slight machining deviations on the extension shoulder itself which exceeds allowances between exact and nominal dimensions. There are other factors as well.

  4. #124
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    Years ago I had only a Colt 6920. I zero'd the glass on it and shot it alot.

    About a year later I wanted to see how a rifle with a mid-length gas system shot. I put together an upper using a BCM upper receiver, DD CHF 16" LW profile barrel, and a URX II.

    Once together I swapped the glass from the 6920 to the mid-length upper. Re-zeroing took many clicks of elevation and windage. Once I was done shooting the mid-length upper I would swap the glass back to the 6920 upper and re-zero. About two months later I learned about the tool from Brownells. I bought it and lapped both receivers. I was still swapping the glass at that time. After lapping the windage setting for both uppers did not need to change. I still had to change the elevation but it was not as dramatic.

    That is my experience. YMMV.

  5. #125
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    Quote Originally Posted by EzGoingKev View Post
    Years ago I had only a Colt 6920. I zero'd the glass on it and shot it alot.

    About a year later I wanted to see how a rifle with a mid-length gas system shot. I put together an upper using a BCM upper receiver, DD CHF 16" LW profile barrel, and a URX II.

    Once together I swapped the glass from the 6920 to the mid-length upper. Re-zeroing took many clicks of elevation and windage. Once I was done shooting the mid-length upper I would swap the glass back to the 6920 upper and re-zero. About two months later I learned about the tool from Brownells. I bought it and lapped both receivers. I was still swapping the glass at that time. After lapping the windage setting for both uppers did not need to change. I still had to change the elevation but it was not as dramatic.

    That is my experience. YMMV.
    Interesting results. Did you note any other changes such as improved accuracy? The reason I ask is that you hear people make the claim that turning the face improves accuracy. In your case, it obviously improved centricity however, it would be nice to know if you noticed any other positive changes.
    “I predict future happiness for Americans, if they can prevent the government from wasting the labors of the people under the pretense of taking care of them.”
    Thomas Jefferson

  6. #126
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    Quote Originally Posted by bp7178 View Post
    Have you tested this or just come up with a hypothesis? Also, in your 0.004" example which results in 3 clicks of windage; what length sight radius, what is the distance the rifle was zeroed at, and what is the click value?

    It would be interesting for someone who had the resources, to take a upper and purposely make it out of square and test. Its a pretty common complaint whereas users will need what they consider an excessive amount of windage. I don't think I've ever read anything definitive about it, just a lot of internet bickering.
    Unless your upper receiver and barrel extension reside in a plane of existence where the rules of trigonometry do not apply, or two different objects can coexist in the same space, my calculations are pretty darn ‘definite’.

    And, you don’t need to purposely destroy an upper to show this is the case. With a barrel inserted in the upper all the way to seat on the flange, without a barrel nut, or the nut not engaged on the upper’s threads, gently try moving the muzzle from side to side. Try to keep from bending the barrel.

    Twelve (12) clicks of windage, with 32 pitch threads on the windage screw equals 1/3 inch.

    If the barrel is loose enough in the upper to allow the front sight to move more than 1/3 inch, then it is seriously out of dimensional tolerance. If it does not move that much, measure the extreme movement, I’ll bet it is at most a few thousandths of an inch.

    Oh, and about the sight radius - think about it. It doesn’t matter.

    If the barrel is pointed off to the side and it takes 15 clicks of left windage, or .417 inch, to correct it, how far to the right is the front sight?

    For illustrative purposes let’s look at this exaggerated example below. Print it out and draw where the rear sight has to be in order to zero this barrel with the front sight in position #1, now doing again for position #2 . . . Does the position of the rear sight move with a different front placement?

    Nope.


    (Note: we are looking down from above the rifle with an extreme mis-alignment.)

    Now, how much is the rear sight offset?

    It is the tangent of the angle between the center-line of the upper receiver and the barrel axis multiplied by the distance from the rear sight to the bend. And, the angle of the bend is the arc-tangent of the distance the front sight is offset divided by the distance from the bent to the front sight.

    Play with the math as much as you want, but the rear sight offset will always be less or equal to the front sight offset, except for pistol length barrels. And even with those, unless the distance from the extension flange to the front sight is less than three (3) inches, the rear sight offset will be the same to one or two thousandths of an inch.



    In my above posts, I used the worst (realistic) case where the rear sight offset was the same as the front sight offset.

  7. #127
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    Quote Originally Posted by tom12.7 View Post
    There's more variables to look at yet, at this point we as a group are not looking at them all.
    Maybe we can look into what portions of the bore and face of an upper under the barrel nut torque tension can induce?
    Maybe we can look into some of the deviations that occur?
    I will make an analogy -

    If you put a five cent piece (the nickel, the thickest common US coin) on a railroad track rail and a train goes over it, does the train tilt to one side as it passes over the nickel?

    Yes, it does.

    But, is it enough to make any difference at all to the train?

    No, it isn't.

  8. #128
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    Quote Originally Posted by lysander View Post
    Unless your upper receiver and barrel extension reside in a plane of existence where the rules of trigonometry do not apply, or two different objects can coexist in the same space, my calculations are pretty darn ‘definite’.
    If the barrel is not square to even a small degree and then "crushed" in place by the barrel nut, can there be asymmetric load/stress on the barrel root?

    If the answer is yes, can that asymmetric load contribute to barrel harmonics that send bullets further away from the predicted flight path? And then could you argue that changing the torque on the barrel nut should change the harmonics as well if something was not square?

    The anecdote from EZGoinKev seems to point to other forces at play that are influenced by how true the receiver is. And its not just a static trig problem. Just trying to find an explanation to what he saw.

  9. #129
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    Quote Originally Posted by Rayrevolver View Post
    If the barrel is not square to even a small degree and then "crushed" in place by the barrel nut, can there be asymmetric load/stress on the barrel root?

    If the answer is yes, can that asymmetric load contribute to barrel harmonics that send bullets further away from the predicted flight path? And then could you argue that changing the torque on the barrel nut should change the harmonics as well if something was not square?

    The anecdote from EZGoinKev seems to point to other forces at play that are influenced by how true the receiver is. And its not just a static trig problem. Just trying to find an explanation to what he saw.
    Compared to the .0015" allowable perpendicularity tolerance between the bolt lugs and the extension lugs?

  10. #130
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    I would not overlook torque induced distortion over the less tensioned fitting, be it stresses or strains as variable to look into. That doesn't automatically mean that the spec is wrong, but some combinations don't play as well as others can, even with the same static fit.

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