Safety Issue On AR Rifle Building

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  • JeepHammer

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    Moderators, if you find ANYTHING factually wrong with this information, correct or remove it immediately PLEASE!

    Gunsmithing, building accuracy rifles was part of my job in the Marine Corps,
    Marines build, or rebuild their own special weapons, and this is a safety issue that I don't see much of anything posted about.

    Upper receiver are forged or cast by manufacturers.
    Threading the front of that upper, where the barrel mates and a barrel nut holds the barrel in place on the upper MUST be square with the bore of the upper,
    The forging & threading process will distort the mating surface for the barrel.
    99 times out of 100 YOU will have to square this surface.

    On the left, a new, forged, milspec upper receiver.
    On the right is another new, forged, milspec receiver that's in the process of having the upper/front barrel mating surface lapped (ground) square to the bore.
    This is so you know which surface needs correction...

    IMG_00601_zpslmirkfco.jpg


    The above picture shows the high spots being worn away first (shiny) while the other side isn't touched yet.
    With a bar centered in the Bolt Carrier Group (BCG) bore of the upper receiver to guide the material removal exactly square with bore,
    This allows the bolt carrier/bolt to line directly up with the chamber nut locking lugs.

    If the barrel is cocked sideways, or an air gap under part of the barrel mating surface,
    The bolt lugs simply CAN NOT engage the chamber nut squarely, and the firing pressure will be transferred to one or two lugs.
    This is a VERY good way to sheer the chamber nut or bolt lugs... VERY DANGEROUS putting that much pressure on just one or two lugs.

    This is a picture of the barrel mating surface completely lapped (ground) square (90*) to the center line of the upper receiver bore.

    Uppers02_zpsk35yh9bq.gif


    Notice the polished ring isn't the same width all the way around...
    I only take enough off to square this surface with the bore.

    *IF* you have chambering issues (too much needed to be removed) then shims are commercially available and perfectly acceptable to use. This is actually the way most high accuracy rifles are adjusted for the Nth degree adjustments.

    The easiest tool for facing this surface off is a commercially available, and inexpensive gunsmith tool called (ironically) a lapping bar.
    Just measure the bore shank and make sure the one you get doesn't exceed the maximum diameter of upper specifications.
    You don't want to oversize the BCG bore while facing off the upper...

    In this picture, you see lapping compound (grit) for the cutting g surface,
    Lubrication for the pilot bar that fits into upper receiver, and the lapping tool itself (stainless steel bar).
    Above are untouched & finished upper headed for accuracy builds.

    UppersTools_zpsy0zzgr9v.gif


    By aligning the BCG with the barrel/Chamber nut, the locking lugs can make contact, keep contact all the way around during firing.

    The locking lug issues usually present as failure to eject, failure to lock into battery instead of catastrophic failure, but don't discount catastrophic failure.
    As lugs with the most pressure applied to them, they flatten, deform, mushroom, and sometimes sheer off.
    The added deformation often shows up as malfunctions, from added friction and poorly fitting parts, it's an indicator the lugs are failing.

    -

    Since I make money gunsmithing, I don't normally give away tips on how to build a better rifle, but this is a safety issue that's EASY to correct by even a home builder.

    *IF* you are going to forego a qualified gunsmith (not recommended) the lapping bar & lapping compound are inexpensive & commonly available in any 'Gunsmith Tools' section of major suppliers.
    You can hand turn the lapping bar, or use a LOW SPEED drill motor, and it takes less than 30 minutes even by hand.
    Lubrication on the guide, lapping compound under the head, rotate until you get a polished ring all the way around -- done.
     
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    NyleRN

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    In theory you're correct but for all practical purposes in the field it doesn't matter if you lap your upper. Nobody hardly does that and I'm not hearing, seeing, reading of folks shearing lugs off left and right. IMO, it's a solution looking for a problem
     
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    T755

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    Built a few thousand Ar platforms. Yes for precision DMR builds I square and lap. Average AR's no. I just watch the wear pattern on the lugs as they come thru for preventive maintenance as you do with any ar as it ages. In 20 years I've seen a grand total of 2 sheared lugs. One was a high count swat gun and a twice ANAD rebuilt M16A1 on its probably 2nd barrel. I wouldn't lose sleep either way.
     

    Clay Pigeon

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    As a guy who has done hundreds of repairs to injection molds up to 20 tons I know that truing one surface of a two surface union doesn't guarantee perfect mating.
    There is interference between the threads on the barrel nut and the receiver, the threads on the barrel and receiver and the shoulder of the barrel and the lapped front of the receiver. You want a perfect union, lap both mating surfaces together. But like others have posted, for the most part I wouldn't waste my time.
     

    JeepHammer

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    Built a few thousand Ar platforms. Yes for precision DMR builds I square and lap. Average AR's no. I just watch the wear pattern on the lugs as they come thru for preventive maintenance as you do with any ar as it ages. In 20 years I've seen a grand total of 2 sheared lugs. One was a high count swat gun and a twice ANAD rebuilt M16A1 on its probably 2nd barrel. I wouldn't lose sleep either way.

    What I see a LOT of is one or two lugs battered down, mushroomed or smeared and a couple more starting to take up the load.
    Every single time it's the barrel cocked sideways...

    The complaint when I see them is failure to lock up, misfire, having to beat on a forward assist to get it back into battery.
    Some stuck bolts, the 'I had to fire the chambered round' to a lesser extent.

    There is a reason the AR bolts have more than one or two lugs...
    You have to sheer all lugs to split the BCG/upper, but when the one lug that was in direct contact sheers,
    It's then MOVING sheer load against the others... Momentum.

    I get paid for accuracy, so the benefits to consistency/accuracy I don't get into...

    My point is, there is a potential for catastrophic failure built into the rifle, even if it's small.
    How many accidental discharge stories do you know about while someone is trying to get a firearm into battery or clear a live round jam?
    That's a safety issue...
     

    JeepHammer

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    As a guy who has done hundreds of repairs to injection molds up to 20 tons I know that truing one surface of a two surface union doesn't guarantee perfect mating.
    There is interference between the threads on the barrel nut and the receiver, the threads on the barrel and receiver and the shoulder of the barrel and the lapped front of the receiver. You want a perfect union, lap both mating surfaces together. But like others have posted, for the most part I wouldn't waste my time.

    Do you know how much pressure is on a bolt that's less than one square inch in an AR?...

    Lapping lugs usually isn't a huge deal on ARs since the bolt & chamber nut are lathe turned and true.

    You *Can* lap AR bolts & chamber nuts, but that's for the Nth degree headspace/lockup accuracy, and I don't often talk about how to do accuracy/consistency improvements.
     

    JeepHammer

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    If you have a bbl nut that is on the high side of torque and still not lining up......lapping should drop it a bit.

    Lapping/shimming solves a lot of problems with fitting and aligning, getting the rifle to run smoothly & reliable...

    Getting the BCG & barrel aligned is a big headspace deal also, another safety issue.
     

    Clay Pigeon

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    Do you know how much pressure is on a bolt that's less than one square inch in an AR?...

    Lapping lugs usually isn't a huge deal on ARs since the bolt & chamber nut are lathe turned and true.

    You *Can* lap AR bolts & chamber nuts, but that's for the Nth degree headspace/lockup accuracy, and I don't often talk about how to do accuracy/consistency improvements.

    LOL... Do you really believe that FN's lathes cut the same and Rock Rivers, White Oak and the dozens of other AR builders in the US and abroad?
     

    JeepHammer

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    LOL... Do you really believe that FN's lathes cut the same and Rock Rivers, White Oak and the dozens of other AR builders in the US and abroad?

    Yes, I do.
    The function of all lathes is to cut round & square.
    The tolerances of lathes will differ depending on condition & maintenance, but round & square cuts are what lathes do.
    It's their basic function.
    It's the reason lathe tooling moves straight & square with the material being turned...

    It would be massively difficult to cut something on a lathe 0.075" out of square by a professional, but to have uppers 0.075" to 0.100" skewed.
    I also know the big manufacturers have CNC equipment that automatically checks for square/true, and major manufacturers do QC on everything.

    It's the home builder that might not know their square/true parts aren't lining up because the upper they purchased doesn't have a square/true front facing.
    The big manufacturers will square the uppers before assembly, because they are professionals and understand how things are supposed to work.
    Home builders aren't professionals by definition...
     

    JeepHammer

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    To the OP, thank you for taking the time to post this information. You never can know enough and there is always more to learn.

    No problem.

    It's common sense...
    You wouldn't intentionally build an engine out of balance, you don't intentionally get 'U' joints or drive lines in cocked sideways,
    You don't bend one side of a lawnmower blade intentionally...

    I've seen nearly zero written about it, so people just don't know about it.

    Clay Pigeon brought up the question of squaring the lugs, it's pretty easy to check the bolt, you slap a square on it.

    Some barrels/chamber nuts are suspect, so I check them also, takes a hot 10 minutes, a simple fixture & dial indicator to see if the chamber nut was cut true and on the barrel true with bore...

    IndicatorLip_zpsxpqpexde.gif


    Once the barrel is hung between centers so you are indexing from the bore, it's easy to check inside & outside of the chamber nut.

    No point in building a Minute Of Paper Plate when it can shoot sub MOA on exactly the same parts.
    No point in building something that glitches when it can run smooth & trouble free on the same parts...
     
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    Clay Pigeon

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    Yes, I do.
    The function of all lathes is to cut round & square.
    The tolerances of lathes will differ depending on condition & maintenance, but round & square cuts are what lathes do.
    It's their basic function.
    It's the reason lathe tooling moves straight & square with the material being turned....
    .

    I'm well aware of what a lathe is for.
    Since you believe that all lathes cut true and the big manufactures all do quality control why is it necessary to lap the upper?
     

    billybob44

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    Thanks for the thread Jeep...

    This was an interesting read for me.
    It just so happens that another INGO Member and I share a Lapping tool with the compound.
    I have done six or so uppers, and my friend has done about the same.
    For us this system works well to help in the alignment of the two surfaces.
    Thanks again for the thread Jeep..Bill.
     

    JeepHammer

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    I'm well aware of what a lathe is for.
    Since you believe that all lathes cut true and the big manufactures all do quality control why is it necessary to lap the upper?

    You just can't help trolling can you?

    If the upper were made on a lathe it probably wouldn't need facing off.
    As I said in the very beginning, threads are hogged onto the upper blank, which distorts the front of that blank.
    If the threads were lathe turned, just one touch of the cutting tool would face off the upper square while it was on the mandrel.

    It's an oversight, a manufacturing quirk, an oddity that occurs...
    If the upper had seen a lathe, it would probably be gone, but cutting threads with a die, or rolling threads on the upper, either way you get the misalignment.

    And continuing to argue with a machinist/gunsmith about the function of machine tools or processes and/or gunsmithing procedure probably isn't the best idea...
    Particularly since others have already said they do it for accuracy rifles, it's known to gunsmiths, but not the general public.
     

    JeepHammer

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    This was an interesting read for me.
    It just so happens that another INGO Member and I share a Lapping tool with the compound.
    I have done six or so uppers, and my friend has done about the same.
    For us this system works well to help in the alignment of the two surfaces.
    Thanks again for the thread Jeep..Bill.

    It's not well known, but easily corrected without machine tools.
    In the Marines, we put the uppers on a mandrel, between centers, and touched the face with a face off tool.
    The lapping bar makes it SO much easier.

    If there wasn't an issue, manufacturers wouldn't make lapping bars or shims commercially.
    It's not just a here or there thing, and it's quite sever on some upper, particularly forgings.

    As the one lug in contact gets hammered/smeared, headspace changes, it opens up, and that can become a serious issue also.
    With all solid lugs in contact, the headspace is maintained much longer.
     
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    Clay Pigeon

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    You just can't help trolling can you?

    If the upper were made on a lathe it probably wouldn't need facing off.
    As I said in the very beginning, threads are hogged onto the upper blank, which distorts the front of that blank.
    If the threads were lathe turned, just one touch of the cutting tool would face off the upper square while it was on the mandrel.

    It's an oversight, a manufacturing quirk, an oddity that occurs...
    If the upper had seen a lathe, it would probably be gone, but cutting threads with a die, or rolling threads on the upper, either way you get the misalignment.

    And continuing to argue with a machinist/gunsmith about the function of machine tools or processes and/or gunsmithing procedure probably isn't the best idea..
    .
    Particularly since others have already said they do it for accuracy rifles, it's known to gunsmiths, but not the general public.

    Arguing????? I'm not arguing at all, I was having a conversation. Sounds like you are the one thats frustrated and wants to argue.

    Where am I Trolling you? You gotta be ****ing kidding.. I answered in a serious manner as a rebuttal and this is what you post....

    I'm quite sure I work on much more expensive ( up to 350,000 dollars and the size of a sub compact car) and i'm one of those guys who tigs parting lines in molds and will hand file / polish damage done to a mirror finish mold with both sides being A parts that have to have much more more stringent measurements then a custom AR will ever be.
    I'm out, Good Luck...
     

    cbhausen

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    The bolt is what, 1/3 the diameter of the barrel nut? So any runout at the front of the upper receiver is cut to approximately 1/3 at the bolt locking lugs, right?

    I understand and appreciate those with a passion for precision but for the vast majority of AR-15 builds just bolt it together and it will run fine (check and monitor the headspacing, of course).
     

    JeepHammer

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    The bolt rotates.
    Either you understand the bolt and chamber nut lugs need to be on the same plane of motion, or you don't...

    Either you want headspace to open as one lug gets hammered, or you don't...
    Either you want the bullet to cross line of sight on the horizontal plane, or you don't...
     
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