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

  1. #81
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    I have seen even the best forged and billet upper receivers benefit from this.
    If you can insure a more uniform contact between the bolt and the extension what is the negative?
    I look at it as "blueprinting" like an engine.
    Just because something falls between manufacturing tolerances does not mean it cannot be improved by tightening up the slop allowed in the manufacturing process.
    With the amount of money we spend on these weapons the question isn't why would you, it's why wouldn't you.

  2. #82
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    Quote Originally Posted by Robb Jensen View Post
    T. On a few I've heated the end of the upper with a heat gun before to get a barrel to seat well.
    I think this is a perfect fit.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  3. #83
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    This is an LR-308 upper that has been face lapped, we had to make the lapping tool ourselves.
    This will be the second upper I have done, it does make a difference.

    IMG_1830.jpg
    Last edited by smihtp; 01-12-16 at 12:08.

  4. #84
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    The main bore that those lapping tools ride in SHOULD BE larger than the extension bore. If they are small enough to get through the extension bore, to ride in the main bore, then you are not lapping your receiver face square to anything. Your hand drill and lapping tool is not as accurate as the CNC that was machining it! I keep seeing pictures of anodizing taken off just one side.... Because your lapping tool is canted in the sloppy bore. Or it was held up right in a CNC mill one at a time in a garage some place.

    If it was fixtured and machined properly, then those lapping tools could be doing more damage than good. You might see some anodizing coming off one side as it takes itself off axis from slop, and think "Man! This things jacked up!" But most likely you are damaging what was a perfectly in square part with your hand drill and "Lapping" tool.

    I would say, just be aware of exactly what is going on before deciding to do this process.

    We do all the threading, facing, boring, and finishing in one hold off of the rail in order to keep everything as aligned as the Makino A51 can make it. I know some people stand these uppers up in verticals though.

    just something to think about before getting after your upper!


    Ryan
    Founder and co-owner/operator at 2A-Armament

  5. #85
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    I totally respect what your saying Ryan, and yes one should be careful.
    My tool has zero slop and is relief cut before the extension bore, I've NEVER used a drill motor. I lap by hand and only take off what is needed to accomplish the goal.
    I have seen some lapped uppers that IMHO have had way to much material removed, Getting carried away can lead to over extension and serious feed ramp issues.
    Ryan is correct, know what your doing before you dive in.

  6. #86
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    Here is why I do not subscribe to truing the face/bore with the usual assortment of available lapping tools:

    The loss of accuracy with the AR's barrel retention method is if the front face of the upper, the flange on the extension and the inner flange on the barrel nut are not all parallel to the crush thickness of the softest part, the upper. If these are not parallel, one place on the barrel extension flange gets pinched between the upper and nut and the barrel extension flange can move slightly.

    There are a few ways these things can get out of parallel, the face isn't square to the threads and/or the threads aren't parallel to the bore, the extension flange is not a uniform thickness, or the nut threads aren't parallel to the nut seating flange.

    1) Lapping the face relative to the bore assumes the threads are parallel to the bore, since facing, threading and boring are done off the same set-up, why would the face be way off and the threads be true?

    2) Lapping does not fix the other possible causes of non-parallelism.

  7. #87
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    Lysander, sir you just described why people lap and fit in the first place.
    I want two things in this area to come together on a build, a snug fitting extension and a good flange area on both components.
    When these things come together I feel I have quality components in hand.

    Your absolutely right it doesn't fix all the possible causes of non-parallelism in any upper, I don't feel I implied otherwise.
    But it does help address one. That one area may be good enough for a budget builder, and also for Brownells and Sinclair to manufacture and sell the tool.
    http://www.brownells.com/gunsmith-to...prod20220.aspx Apparently there's a market for it. No one made such a tool for the AR-10 at the time of my build, I simply stated that we made our own. I never presented it as a cure-all in scope of my post.

    We have to rely on the manufacturer to get the threads parallel to the bore, Lapping never assumes the face is "Way Off"? I do not assume that. Its a process to simply refine the mating surface of an upper nothing more.
    Anything that's mass produced roughly handled bead blasted then Anodized is subject to dings and scratches and impurities. I'm sure you have bins full of "Blems" but do they catch them all?

    Please excuse my OCD!
    Last edited by smihtp; 01-13-16 at 20:16.

  8. #88
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    I only use BCM or "better" uppers and I true all mine but I don't think it makes much if any difference. I built 2 identical SBRs and forgot to true one of them. It turned out that it out shot the rifle with the trued upper. I don't think it can hurt unless you get some compound down inside the upper while polishing and lap off the hard coat inside the upper.

    Just sayin.
    Last edited by Sparky5019; 01-22-16 at 17:20.
    "An opinion solicited does not equal one freely voiced," Al Swearengen, Deadwood 1877.

  9. #89
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    I do agree with your experience, it really does not do much other than assure you there are no surface imperfections on the upper face. Yes 95% of the uppers we buy are machined perfectly.
    And as noted earlier in this post, soft metal will form fit itself to the extension flange regardless. Is there a degree of soft metal distortion that occurs during the torquing process, absolutely there is.
    For example: Look at a Rem 700 receiver and barrel, what do bolt gun builders strive for in truing them? Everything seems based off the original center line of the receiver, receiver threads are re-cut and trued to the center line,
    recoil lug and lug flange, all trued to the center line of the receiver. In the AR there's one mating surface that performs the same way, the barrel nut and flange. For all intensive purpose our threads and barrel nut are only a clamp.
    Any way you look at it that single mating surface between barrel and upper is critical. I'm not building a patrol rifle or even a DMR, I'm trying to build a long range precision rifle capable of spanking the bolt gun guys at our range Match's.

    I shoot a modified LR-308 with a custom heavy SS 24" Bartlein barrel, Match BCG and Geissele NM-R trigger (2.5Lbs total) and Hydraulic Buffer, I can easily see my own hits at 800 yards, and yes my Rifle weighs 16 Lbs (I don't care).
    I required a sub .5 MOA "consistent" rifle to compete with my buddies $6K Bolt guns. Why? Because knew I could do it...
    --------------
    My tool has a relief cut about 3/4" before the lapping surface of the tool, this relief does not allow any compound to enter inside the upper extension wall.
    The only lapping contact point is the face.
    Last edited by smihtp; 01-22-16 at 13:07.

  10. #90
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    Quote Originally Posted by lysander View Post
    Here is why I do not subscribe to truing the face/bore with the usual assortment of available lapping tools:

    The loss of accuracy with the AR's barrel retention method is if the front face of the upper, the flange on the extension and the inner flange on the barrel nut are not all parallel to the crush thickness of the softest part, the upper. If these are not parallel, one place on the barrel extension flange gets pinched between the upper and nut and the barrel extension flange can move slightly.

    There are a few ways these things can get out of parallel, the face isn't square to the threads and/or the threads aren't parallel to the bore, the extension flange is not a uniform thickness, or the nut threads aren't parallel to the nut seating flange.

    1) Lapping the face relative to the bore assumes the threads are parallel to the bore, since facing, threading and boring are done off the same set-up, why would the face be way off and the threads be true?

    2) Lapping does not fix the other possible causes of non-parallelism.
    Barrel extensions are machined in one op on a lathe. A 4-8ft 1-3/16" bar is fed through the spindle and held. The bore is drilled, the threads are single point cut, the outside profiled and the flange cut all in one operation, one setup then the the piece is cut off at the rear where the chamfer and feed ramps are. All of those areas should be square with each other. The only part that is not done is the broach and the chamfer on the backside, neither of which matter to the area we are discussing.
    The barrel threads and chamber are also cut in one op, one setup. The chamber must be cut concentric with the bore, the threads concentric with the chamber if done in the same op. Threads are single point cut, a threading die is not used.
    When extensions and barrels are machined in a lathe the material/object spins, the tool is stationary. That is why all parts are square with each other unless the machine is out of adjustment. I could see that on a manual lathe but not on a CNC, they must be checked too often to allow that to happen.
    When receivers are machined they are held stationary and the tool spins/moves. If the receivers are held on a pallet system and cut on a horz mill the tools come in from the side. The same tools are used for many different cuts on the same piece. IF the 1" bore of the receiver is drilled during that operation and then a large diameter facing bit apx 1-1/4" in diameter is used to face the front with the center being held as the same as the 1" bore drill the face of the receiver could be square but I'll bet a 1/2" end mill is used to pass by the front of the receiver while making other cuts on the receiver. We do not machine upper receivers but I have seen quite a few being machined.
    This is where I think most miss the squaring difference. IF...IF there is play between the barrel extension and bore of the receiver and the face of the receiver is not square with the bore of the receiver when the barrel nut is tightened down it will cock the extension inside the receiver. The extension flange will be flattened to the face of the receiver and the OD of the extension will not be square upon the bore of the receiver.
    IF the extension IS a tight fit in the bore of the receiver I don't think the flange can cock the extension in the bore. The extension will be aligned by the tight fit of the OD to the bore not the flange so there is minimal to gain by squaring the front of the receiver but, it can't hurt.
    Some receivers are very close maybe .002" side to side. After squaring thousands over the last 8 years there may have been 3-4 come through that were perfect. Most run .003" to clean them up across the 1" bore. Mega Arms and Vltor receivers have always had a tight bore. They square the extensions and provide a good rigid fit and that is why I have always recommended them. Over the last 2-3 years I have seen a few other brands that are tight. The last batch of receivers from Aero P fit tight to the extensions we use.
    Last edited by constructor; 01-22-16 at 13:24.
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