AR-15 Barrel Mounting - Help Me Decide What to Use

Discussion in 'Gun Project Questions & Gunsmithing' started by skeeljc, Dec 8, 2019.

  1. GenePoole

    GenePoole

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    I'm just curious here as I don't know much about ARs. Why is the upper 2 thousandths bigger than the barrel extension (or is the extension 2 thousandths smaller)? All the AR's I've dealt with are almost an interference fit, requiring a few thumps from a rubber mallet to seat the barrel into the upper.
     
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  2. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    0.002 is on the sloppy side. It should be a tight fit. One of the two parts (or both) are a tad out of spec most likely. Some guys use loctite 620 and/or steel shims to fill the gap as needed.
     
  3. akajun

    akajun

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    order a ss shim stock pack from one of the machine shop supply houses Like Mcmaster carr or from Brownells. You will not more than likely be able to completely wrap the barrel extension with the shim and have it still fit, and that's ok, your just taking up space to minimize movement of the barrel in relation to your sighting device. Ive played around with this and yes it does make a small, say 20-30% improvement in group size but its still not gonna turn a turd into a benchrest gun. Best it ever did was turn a 1.5" to 1.25" gun into a 3/4" to 1"gun which for Service rifle is all I need.
    While your at it make sure that the upper receiver face that touches the barrel extension shoulder is flat. Sometimes they are not or there is a burr and that can cause issues. I made a little lapping tool that I use to check it with layout dye. If you do have to lap, only do enough to take out the high spot, don't remove the anodizing all the way around. If you don't have a lathe X caliber barrels make the BART tool which is the same thing.
    As far as minimizing movement between the upper and lower, it may make you feel good to fix it as its is somewhat annoying at first but it makes no difference in accuracy. I stopped using the little accuwedges because it made it harder to take them apart for a quick repair while slung up. I don't even notice the movement anymore when shooting. Years of service rifle shooters have tried everything from shims to even bedding the receivers together, no one does any of that anymore.
    Id be careful about letting anyone ream the pivot pin holes for oversized pins. You are removing the very thin layer of hardcoat anodizing when doing so and will accelerate wear in that area, then causing more of the problem you are trying to fix.
     
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  4. spitfire_er

    spitfire_er

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    There is a good article on this site about Robert Whitleys AR 15 rifles and he is extremely knoledgeable when it comes to AR accuracy. Last I saw he was using the DPMS low pro's that he modified a little.

    That's what i use. If you are turning your own barrel best thing to do is to get a BAT barrel extension that fits your reciever. Next best IMO is a tight SS shim. With the correct shim you can get a very snug fit. Careful cutting will allow a shim all the way around the barrel extension. While many people have great success with loctite, to me it's a mess.

    For upper to lower fit, I use carefully placed black electrical tape. Cheap, easy, and lasts a long time. With good fitment between upper and lower and a heavy walled upper, that's about all you can do to minimize flex. I haven't measured flex in mine, but would bet it's very minimal if any. I believe it would flex on a standard upper quite a bit more.
     
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  5. skeeljc

    skeeljc Silver $$ Contributor

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    I appreciate both of the above comments from "akajun" and "spitfire_er". Good information.

    I will try to shim the gap between the barrel extension and the receiver. My gap is .002" and I have .001" thick ss shim stock.

    My thought on the movement between the upper and lower receivers is when the hammer strikes the firing pin the upper may move relative to the lower which could affect accuracy. I suppose the weight of the upper assembly plus the downward pressure on the stock from the shooter's cheek weld might negate any movement of the upper assembly from the hammer strike. The Geissele High Speed NM trigger has a fairly low mass hammer which also helps.

    I too feel better removing the play between the receivers. My solution is to ream the front pin holes to .250" diameter and use a .2495" diameter shoulder bolt with a brass nut. This makes a tight connection and can be torqued to eliminate a small amount of side play. I then ream the rear pin holes (upper and lower together) for an oversized pin. My Aero Precision lower has a screw with a polymer tip that pushes up against the pin boss of the upper. I have to back this screw off to move the rear pin for cleaning.

    I will mount the upper on a 1" shaft between centers in my lathe and check for run out on the receiver face. I expect with today's CNC machines that there will not be much.
     
  6. milanuk

    milanuk Gold $$ Contributor

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    You might be surprised...

    20191204_221035.jpg
     
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  7. jamesh

    jamesh

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    Dammit! Now I am going to have to start checking my AR receivers.
     
  8. Bamban

    Bamban Silver $$ Contributor

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  9. Fast14riot

    Fast14riot Silver $$ Contributor

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    I haven't had one that didnt need some amount of lapping.
     
  10. skeeljc

    skeeljc Silver $$ Contributor

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    The DPMS Lo Pro upper I have is 1.0020" diameter when measured with a bore micrometer. The barrel extension on my White Oak barrel is 1.0000".

    The Aero Precision uppers I have used usually measure 1.0010" diameter.
     
  11. jkohler

    jkohler Silver $$ Contributor

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    Seems the standard AR barrel extension is .998 Bat machine makes them .999 and 1.000 I’m not sure if they go bigger than that. I’ve not done a crap load of AR’s but The ones I’ve done the uppers have been .998 to 1.000. I’d get a different upper if they where looser than that.
     
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  12. milanuk

    milanuk Gold $$ Contributor

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    I happened to be browsing on another forum and came across a (somewhat) related discussion (on BCG droop), to which someone had posted links to some articles in 'American Gunsmith' magazine. I started reading, and saw a reference to further articles by the same author on the topic at hand here.

    For those interested, here's a link to the index of articles by that author... should make for some interesting reading!

    P.S. the articles on barrel extension fit are near the bottom
     
    Last edited: Dec 11, 2019
  13. skeeljc

    skeeljc Silver $$ Contributor

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    I set up an Aero Precision AR-15 upper and measured the runout on the receiver face. The TIR is .0025". Is it worth losing the hard anodizing to clean up .0025" of runout?

    20191215_120027.jpg 20191215_120007.jpg
     
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  14. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Can i see that whole setup you have going there?
     
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  15. skeeljc

    skeeljc Silver $$ Contributor

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    It is a 12" piece of 1" diameter TG&P 1045 stock. I put a center in one end and the other end is in the chuck. The Aero Precision upper is a tight slip fit on the 1.000" diameter stock.

    I just received my DPMS Lo Pro upper. I will take another pic when I check it out.
     
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  16. skeeljc

    skeeljc Silver $$ Contributor

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    The DPMS Lo-Pro upper receiver checks out within .0005" TIR runout on the barrel face.

    Here is the complete setup I used:

    20191215_181833.jpg 20191215_181917.jpg
     
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  17. Dud

    Dud

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    Anodizing adds surface hardness and corrosion resistance. Inside a bolted connection that doesn't rotate, that's not really needed.

    So the question is do you care enough about anodizing to tolerate 0.0005"?

    An oversized receiver seems like a good idea until you realize that the threaded snout of the receiver is the weak spot and they are all the same size unless you go to an inverted barrel nut upper...
     
  18. skeeljc

    skeeljc Silver $$ Contributor

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    Well the heavy duty DPMS Lo-Pro upper can't hurt. I am planning to use ss shim stock and 620 Loctite on the fit between the barrel extension and the upper. I think I will ignore the .0005" runout on the receiver face.
     
  19. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    So is the action a slip fit on the bar? How is it secured?
     
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  20. Dud

    Dud

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    Good luck squeezing a 0.001" shim in to a 0.001" gap.
     
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