Longevity, Accuracy Tips for the AR15

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  • dmsims21
    Warrior
    • Nov 2012
    • 430

    Lapping Tool Clearance Sketch

    Lapping Tool Gap.pdf
    www.FriendsvillePrecision.com - AR15 Dry Fire Device

    Comment

    • stanprophet

      Originally posted by dmsims21 View Post
      Some on this forum and elsewhere on the interwebs have questioned the ability of the lapping tool to square the receiver face to the bore.
      It has been noted that the lapping tools are cut to 0.997" and that the receiver bore can be as large as 1.002". That leaves 0.005" of clearance that the tool can be off axis.
      The 0.997" portion of my wheeler lapping tool is 5.5" long. Imagine the tool sitting in the receiver in the worst off axis position. The tool would contact the receiver bore in two places. Connecting the contact points would create a right triangle with a 5.5" leg and a 0.005" leg.

      atan(0.005/5.5) = 0.052deg or 3.13MOA.

      3 MOA isn't that bad. Who has taken a brand new upper and mounted a new scope to it with the windage in the center of its adjustment range? I have and I was off by more the 3 MOA.
      I know there are other tolerances in play, but the worst case clearance won't add a whole lot of error.

      Some important things to note.
      1. That angle only puts the bolt lugs off by 0.00067". There should be enough wiggle in the bolt and carrier to overcome that gap and get even lug contact. (Just and educated assumption on my part.)
      2. Even though the receiver face is non-perpendicular by 3MOA, the receiver face is very flat. The flat face will keep the barrel from rocking back and forth on two high spots.
      3. It is possible that the tool will self-center a little bit with a heavy oil film.

      If I made a mistake, don't beat me up. It is late and I graduated college a long time ago.

      I will post a sketch soon.
      Interesting,
      Here is my take. I think it would be impossible to true the face perfectly. When I test fitted the barrel on the 300, there was a noticeable difference on the barrel centering with the 13 inch rail, it was obvious the barrel was not centered. The 13" rail being so long made the run out so obvious. The upper receiver was a Surplus Ammo upper, supposedly machined by Aero Precision. The barrel extension was a very tight fit into the receiver. The lapping tool to upper fit was also tight, but with a light coat of oil is was turning fine with little bind. After the lapping the barrel was perfectly centered in the rail. When I sighted in the rifle, I used the red dot off my carbine. The carbine was pretty much centered when compared to its rail, and the rear sight was centered for windage. The 300 was only off windage wise 1 MOA. When considering the variance in the top rails, and a cheap mount that was not designed to re zero I think it was pretty good.

      I think you are right, what it comes down to is minimizing the run out on the assembly as a whole. Provide a flat surface for the barrel extension to seat against. Im not an engineer, just a mechanic that is use to taking measurements and making sure of proper clearances and runout on some pretty high tech engines. I could be wrong, but on a receiver face that is not true, I suspect the lack of clamping surface area on the barrel extension would induce some harmonics into the barrel when fired. I could be wrong, someone else here may know better than I do. As far as I see it, there is really nothing to loose by lapping the upper face, and a whole lot more to gain.

      One of my early mentors that use to build some amazing race engines had a big secret. Many guys built the engine to produce the most power, to win races. He would do everything he could to reduce parasitic drag, and heat without hurting reliability. Even at the expense of horse power some times. He told me if you can reduce these things without hurting performance, there is no reason not to. If a little bit of reliability costs a horse or two, then its worth it. HP does not win races, when you are broke down half way through.

      The only draw back I can see with lapping is the removal of the hard coat anodizing, the benefits seem to out weight the draw backs for sure.

      Comment


      • I have had to use oil to even get the lapping tool to spin. Add the thickness of the oil and the .005", fractioned to .0025" gets filled circumferentially pretty quick.

        A lot of handguards will show run out at the fore end, especially the longer ones. That can also be an issue of upper receiver threads not being square to the BCG raceway axis, internal mating surfaces of the handguard-to-barrel nut not being true, etc.

        One smith I ran into at a local range goes full OCD on every surface he can possibly think of on AR receivers, barrel extensions, barrel nuts, and bolts. He would true the upper threads, barrel nut threads, square the barrel nut internal face that abuts to the barrel extension flange, square the barrel nut flange, etc. etc. He was actually testing a 6.5 Grendel he had built when I met him, and the first group out of the pipe was sub 1/2 MOA with a 123gr Lapua Scenar load, so I wasn't going to argue with him.

        Yes, it makes logical sense that having a flat receiver face helps support consistent barrel harmonics, rather than shock-setting the barrel each time you pull the trigger with an unbalanced face, which would lead to erratic grouping.
        Last edited by Guest; 03-12-2014, 03:55 PM.

        Comment


        • Originally posted by dmsims21 View Post
          [ATTACH]4832[/ATTACH]
          I did not know one can still find the green engineer's pad! My favorite was tan.

          Comment

          • dmsims21
            Warrior
            • Nov 2012
            • 430

            Originally posted by JASmith View Post
            I did not know one can still find the green engineer's pad! My favorite was tan.
            I love using an engineer's pad. It makes what is written on it apear smarter.
            My brother is getting his Mech Eng Degree at West Point. He gave me a pad with West Point letter head. I reserve that pad for special projects.
            www.FriendsvillePrecision.com - AR15 Dry Fire Device

            Comment

            • ThatRandomGuy
              Bloodstained
              • Dec 2013
              • 38

              Originally posted by JASmith View Post
              I did not know one can still find the green engineer's pad! My favorite was tan.
              My school sells them in packs. Of course, it is a smaller school with over a third of students being Engineering/ETech, so...
              Like dm says, they're great for project sketches/plans

              Comment

              • RakkasanDad
                Bloodstained
                • Jan 2014
                • 49

                Quick question for LRRPF52. After you are through lapping the upper do you get it cleaned up, then blast, then Cerakote over the new Lapped area when you paint the upper? So that the metal isn't exposed. Thanks.
                NRA Life Member and Proud Army Dad!!

                Comment


                • Originally posted by RakkasanDad View Post
                  Quick question for LRRPF52. After you are through lapping the upper do you get it cleaned up, then blast, then Cerakote over the new Lapped area when you paint the upper? So that the metal isn't exposed. Thanks.
                  Yes. The best way would be for the manufacturer (machine shop who does finish machining) to get the face square to the raceway before anodizing. A good Type III hard coat anodizing is hard to beat for surface treatment on aluminum, because it basically case-hardens the aluminum to a fairly substantial depth, which helps prevent dimensional variation, corrosion, abrasion, and damage from impact. A 7075 T6 aluminum upper that has been anodized in that manner will last several generations easily, even in extreme tropical rainforest conditions.

                  Comment

                  • RakkasanDad
                    Bloodstained
                    • Jan 2014
                    • 49

                    Thanks.
                    NRA Life Member and Proud Army Dad!!

                    Comment


                    • cerakote micro-slick?

                      Have you tried Cerakote Mico-slick? I have started putting it on the inside of my upper receivers. Good stuff. I have not put enough rounds through it yet see how it wears, but it is Cerakote.

                      Originally posted by LRRPF52 View Post
                      I do the parts individually with the base coat, to include inside the lower receiver in every nook I can find, inside the magwell, and sometimes inside the upper receiver.

                      I have used blast tanks, sanding, and no roughing of the surface at all on my personal guns to see the difference. I even coated melonited barrels that have a very smooth, hard surface, and couldn't believe that it behaves like a sand-blasted job. My stainless Grendel barrel was the first one I tried without abrasing the surface, and it has not had a single scratch or ding even mark the surface.

                      I still will abrade/roughen the surface if I am doing it for a friend, since NIC calls for it, and I understand the concept of having a rough surface for better purchase and adhesion of the compound ceramic-epoxy/enamel secret sauce. It's pretty standard surface coating 101 to clean and sand a surface for a uniform and stronger coat.

                      Yes, you can bake quality scopes at lower temps, and the one in the pics is baked. I contacted Vortex about this when I heard you could do it, and they confirmed it with me, stating the temp limits of 200F. Since I am at higher altitude, I use a bit lower temp than that of 180F, being that lower atmospheric pressure affects gas purged tubes in high heat differently. I even baked my Vortex spotting scope, and my 1-4x GRSC, no issues after several years of pretty hard use.

                      Comment

                      • stanprophet

                        Originally posted by bwaites View Post
                        Hard to find them calibrated in inch/pounds though in most hardware stores. And if they are, they are generally more oriented towards the automobile scales, 120 inch/pounds vs the 10-40 inch/pounds you use for scopes rings and such.
                        Just as I was re reading this thread, I wanted to further comment on this. While this is generally true, there are exceptions. I use torque wrenches daily in my line of work. Although I am not working on aircraft engines, or DOE reactors I have my torque wrenches checked ofter and calibrated when specs reach near maximum of rated percentile values. Any high quality wrench will actually give you a spec on percentage of accuracy over a given range. Some of the higher end models like snap on are very clear on that. Also many of the torque wrenches of high quality will far exceed the percentage of error between 20-80% of the range. I have a Snap On Split Beam type wrench that is used on a lot of non critical parts that have a range value. The split beam is rated from 50-250 ftlbs, at 4% +/-. I have found that the 70% of that range will average less than 1%. Even some of the best wrenches they make are +/- 1%. These are the wrenches that are the only ones approved by the D.O.E. for Reactor work. Even then the 1% is not a huge variance.

                        The use of a Torque wrench is really a compromise to begin with. The Idea is to be able to measure the fastener stretch, and in some cases that is the actual way I measure proper fastener torque. Of course this is impossible to do with some fasteners, so the compromise is to us a torque specification to try and engineer the proper stretch of a given fastener. In more recent years it has been found that since variances in fasteners, and friction under the fastener head and different designs of torquing tools, that torque values are not consistent. Many of the newer specs give a low value torque sequence, then continue the fastener stretch by using a specified degree.

                        For most application here this is really irrelevant to one extent, because engineering fastener stretch on the MSR is a little overkill. The name of the game for most things is consistency. Just like many people that reload and use a powder charger, it is more about being consistent. The way things are torqued needs to be consistent. So for things like barrel nuts, using a center range of the torque specs, with a crappy wrench will still yield a value well into the specified range. Even a wrench for scope rings, being off a good percentage, will still yield good results since all of the fasteners will be torqued consistent.

                        Just something I wanted to add that may help someone one day.

                        Comment

                        • biodsl
                          Chieftain
                          • Aug 2011
                          • 1714

                          Originally posted by stanprophet View Post
                          Just something I wanted to add that may help someone one day.
                          Like me, today. Great data point Stan. Thanks.
                          Paul Peloquin

                          Did government credibility die of Covid or with Covid?

                          Comment

                          • bwaites
                            Moderator
                            • Mar 2011
                            • 4445

                            Originally posted by stanprophet View Post
                            Just as I was re reading this thread, I wanted to further comment on this. While this is generally true, there are exceptions. I use torque wrenches daily in my line of work. Although I am not working on aircraft engines, or DOE reactors I have my torque wrenches checked ofter and calibrated when specs reach near maximum of rated percentile values. Any high quality wrench will actually give you a spec on percentage of accuracy over a given range. Some of the higher end models like snap on are very clear on that. Also many of the torque wrenches of high quality will far exceed the percentage of error between 20-80% of the range. I have a Snap On Split Beam type wrench that is used on a lot of non critical parts that have a range value. The split beam is rated from 50-250 ftlbs, at 4% +/-. I have found that the 70% of that range will average less than 1%. Even some of the best wrenches they make are +/- 1%. These are the wrenches that are the only ones approved by the D.O.E. for Reactor work. Even then the 1% is not a huge variance.

                            The use of a Torque wrench is really a compromise to begin with. The Idea is to be able to measure the fastener stretch, and in some cases that is the actual way I measure proper fastener torque. Of course this is impossible to do with some fasteners, so the compromise is to us a torque specification to try and engineer the proper stretch of a given fastener. In more recent years it has been found that since variances in fasteners, and friction under the fastener head and different designs of torquing tools, that torque values are not consistent. Many of the newer specs give a low value torque sequence, then continue the fastener stretch by using a specified degree.

                            For most application here this is really irrelevant to one extent, because engineering fastener stretch on the MSR is a little overkill. The name of the game for most things is consistency. Just like many people that reload and use a powder charger, it is more about being consistent. The way things are torqued needs to be consistent. So for things like barrel nuts, using a center range of the torque specs, with a crappy wrench will still yield a value well into the specified range. Even a wrench for scope rings, being off a good percentage, will still yield good results since all of the fasteners will be torqued consistent.

                            Just something I wanted to add that may help someone one day.
                            Thanks! Good info!

                            Comment

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