AR15 Barrel dimensions, barrel extension? - Page 2
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  1. #21
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    Default Barrel extensions

    I am also looking for extension prints. Especially material, tolerance, heat treat, etc. Everyone I talked with is 12+ weeks out on orders. Looks like it might be reverse engineering time.....

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    Default Barrel extension drawing

    I have a pdf drawing of the barrel extension and would be glad to email you a copy.

    It is to large to upload so let me know if you still need it.

    I just checked with White Oak Armament and they have Barrel Extensions in stock.

    www.whiteoakarmament.com 1-309-376-2288



    Jim C

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    Default Barrel extensions

    Thanks Jim,
    I just got 10 pcs in the other day and I took measurements off a couple that I had. Thanks for the suggestion on WOA. It would be interesting to see if your drawing is close to my measurements tho if you wouldn't mind. I've got mine drawn in .dwg or .dfx if anyone is interested send me a pm.

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    Just an idea . You might try Bushmasters or DPMS . I think both supply parts to Brownells' .

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    PM sent to HMC710 for a copy of the .dwg files. Thanks.

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    Anyone have the specs for the chamfer into the chamber? Can't find it anywhere.

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    Default Chamfer

    I don't know what the 'factory' spec is, but I set my tool at 75 deg and put a .060" wide chamfer.

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    Thanks guys.

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    Default

    Heres a M4 profile:
    http://www.ar15barrels.com/tech/colt-m4.jpg

    Look in this directory, I thought I found a 9mm barrel profile in there.
    http://www.ar15barrels.com/tech/

    Jon

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    The M4 drawing shows the shoulder that the gas block butts up against as being 6.75" from the front of the barrel extension.

    On AR15barrels he shows the distance from the same shoulder to the breech as being 7".

    I think there is more than .25" from the breech to the front of the barrel extension?

    Does anyone know which is correct?

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    Default Extension barrel thread

    This is the method I use. Please verify your operation with your standards to ensure your safety.
    The breach of the barrel is turned to a dia of 1".
    The thread section for the extention is turned to a diamter of .805" to a lenghth of .624".
    The thread is then cut at 16 tpi.
    The extension is threaded on and torqued in place. I do not have the value handy so verify this, but I think it is in the 30-40 ft lbs range. Please verify for yourself.
    The bolt is then checked for fit and function. I hand fit bolts so they just drag on the engagement and are not loose while engaged.
    To adjust engagement either take more at the shoulder or more at the breach.
    The barrel is then ready for chambering.

    WARNING: Do not use this measurement to accurately and safely install an extension.

    Using a depth gauge I get approximately .500" from the end of the extension to the base of the barrel breach end.

    WARNING: Do not use this measurement to accurately and safely install an extension.

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    Quote Originally Posted by HMC710 View Post
    This is the method I use. Please verify your operation with your standards to ensure your safety.
    The breach of the barrel is turned to a dia of 1".
    The thread section for the extention is turned to a diamter of .805" to a lenghth of .624".
    The thread is then cut at 16 tpi.
    The extension is threaded on and torqued in place. I do not have the value handy so verify this, but I think it is in the 30-40 ft lbs range. Please verify for yourself.
    The bolt is then checked for fit and function. I hand fit bolts so they just drag on the engagement and are not loose while engaged.
    To adjust engagement either take more at the shoulder or more at the breach.
    The barrel is then ready for chambering.

    WARNING: Do not use this measurement to accurately and safely install an extension.

    Using a depth gauge I get approximately .500" from the end of the extension to the base of the barrel breach end.

    WARNING: Do not use this measurement to accurately and safely install an extension.
    Thanks. I got it all threaded and chambered. I just need to torque the extension and pin it.

    The measurement I was looking for was the distance from the front edge of the barrel extension to the back edge of the FSB/gas block (all of the above was still usefull!!). I used the one on the M4 barrel blueprint and it worked fine. Handguards are VERY tight. I used 6.75". I think another 10-20 thou wouldn't hurt.

    All I have left is to drill the gas port, torque the extension and pin it. I am assuming from the length of the pin that I will have to drill first. Is there a common number that anyone uses or should I just measure in each different case?

    Hopefully I will have it out to the range by next weekend to try it out.

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    Default Barrels

    www.ar15.com is a great resource. This is a link to specific torque values: http://www.ar15.com/forums/topic.html?b=3&f=4&t=315921

    Good job on chambering, just make sure you test function with a dummy round after you get the extenion on that the round will chamber and extract manually. As far as the extension pin, I have seen them range from installed high to low on other barrels. Its not very long so it doesn't take much of a pilot hole to sink it into the extension threads. Remember you want to accomplish two things with the pin; 1. Secure the extension. and 2; Index the barrel. Too low a pin will allow the barrel to become mis-aligned in the receiver.
    I do not know if the "factory" drills a pilot hole or not or just presses the pin in and lets it deform the threads, helping to lock the extension in place. I would guess this. I do know that a .050" deep pilot hole really sinks the pin in.

    As to your original question, I took it as "from the breech to the front of the extension"

    The gas block location can vary, to a degree. A 1/4", on a 5.56, is not going to make or break it. If you study the operation of the AR type of gas impingement system, you will be amazed at its complex simplicity. As they say, "timing is everything". Have you measured a factory barrel? I am looking for my barrel port size/location file and will link it when I get it.

    Brl Length (in) Brl Dia (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
    11.5 .625 3.850 .081 .089
    11.5 .750 3.850 .086 .094
    14.5 .625 8.375 .063 .078
    14.5 .750 8.375 .070 .086
    16 .625 8.375 .063 .078
    16 .750 8.375 .070 .086
    20 .625 6.875 .086 .093
    20 .750 6.875 .093 .096
    24 .825 N/A .089 .089

    table spacing did not show up correctly: you are on your own for that
    Last edited by HMC710; 08-10-2009 at 07:26 AM. Reason: added table

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    Quote Originally Posted by HMC710 View Post
    I am also looking for extension prints. Especially material, tolerance, heat treat, etc. Everyone I talked with is 12+ weeks out on orders. Looks like it might be reverse engineering time.....
    I have a copy of the print with material, tolerances, and heat treat specs. I can't figure how to post a PDF, so PM if you want a copy.

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    From Randall (AR15barrels.com),

    I have measured and documented dozens of different factory barrels to include diameters and lengths of every step on the whole barrel.

    From the barrel torque shoulder where the barrel extension torques against, it's 6.75" for carbine, 8.75" for mid-length and 12.125" for rifle to the step at the rear of the gas block seat.
    Breach face is 3/4" back from the torque shoulder.
    Gas port is usually 0.275" to 0.295" forward of the rear step, depending on if there is a handguard cap in the design or not.
    You should be able to figure everything else out from that...


    Unknown, but sounds good:


    On a 20" service rifle barrel the critical dimensions are as follows:

    Length from tang shoulder to gas block shoulder: 12.150" (That's the 1.000" diameter)
    (from there the barrel shoulder is angled back at 10* for 0.85"

    The gas port is 0.310 from the barrel shoulder (0.310" + 12.150" = 12.460 from barrel tang shoulder) *The diameter of that section is 0.75". *The 0.75" diameter is 1.875" long. *The rest of the barrel is 0.740"

    End is threaded 1/2-28 for .600 from muzzle. *

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    Quote Originally Posted by HMC710 View Post
    www.ar15.com is a great resource. This is a link to specific torque values: http://www.ar15.com/forums/topic.html?b=3&f=4&t=315921

    Good job on chambering, just make sure you test function with a dummy round after you get the extenion on that the round will chamber and extract manually. As far as the extension pin, I have seen them range from installed high to low on other barrels. Its not very long so it doesn't take much of a pilot hole to sink it into the extension threads. Remember you want to accomplish two things with the pin; 1. Secure the extension. and 2; Index the barrel. Too low a pin will allow the barrel to become mis-aligned in the receiver.
    I do not know if the "factory" drills a pilot hole or not or just presses the pin in and lets it deform the threads, helping to lock the extension in place. I would guess this. I do know that a .050" deep pilot hole really sinks the pin in.

    As to your original question, I took it as "from the breech to the front of the extension"

    The gas block location can vary, to a degree. A 1/4", on a 5.56, is not going to make or break it. If you study the operation of the AR type of gas impingement system, you will be amazed at its complex simplicity. As they say, "timing is everything". Have you measured a factory barrel? I am looking for my barrel port size/location file and will link it when I get it.

    Brl Length (in) Brl Dia (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
    11.5 .625 3.850 .081 .089
    11.5 .750 3.850 .086 .094
    14.5 .625 8.375 .063 .078
    14.5 .750 8.375 .070 .086
    16 .625 8.375 .063 .078
    16 .750 8.375 .070 .086
    20 .625 6.875 .086 .093
    20 .750 6.875 .093 .096
    24 .825 N/A .089 .089

    table spacing did not show up correctly: you are on your own for that
    Excellent resource. Thanks.

  17. #37
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    Quote Originally Posted by pogo View Post
    From Randall (AR15barrels.com),

    I have measured and documented dozens of different factory barrels to include diameters and lengths of every step on the whole barrel.

    From the barrel torque shoulder where the barrel extension torques against, it's 6.75" for carbine, 8.75" for mid-length and 12.125" for rifle to the step at the rear of the gas block seat.
    Breach face is 3/4" back from the torque shoulder.
    Gas port is usually 0.275" to 0.295" forward of the rear step, depending on if there is a handguard cap in the design or not.
    You should be able to figure everything else out from that...


    Unknown, but sounds good:


    On a 20" service rifle barrel the critical dimensions are as follows:

    Length from tang shoulder to gas block shoulder: 12.150" (That's the 1.000" diameter)
    (from there the barrel shoulder is angled back at 10* for 0.85"

    The gas port is 0.310 from the barrel shoulder (0.310" + 12.150" = 12.460 from barrel tang shoulder) *The diameter of that section is 0.75". *The 0.75" diameter is 1.875" long. *The rest of the barrel is 0.740"

    End is threaded 1/2-28 for .600 from muzzle. *
    Funny. My next question was the same as above, but for mid length. You beat me to it. Thanks!!! I will be building a mid length this week.

    BTW, for anyone interested, the 10.25" that I just built with the help of the great resources on here cylces like a charm and is VERY accurate at 25m. I am going to move out to 100 this week and see how it does.

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    I built a midlength to those dimensions with an, I think, 0.080" gas port. It functions pretty well. I am used to a ~15 lb match rifle, so I left it full diameter under the handguards.

    I made the barrel 17 1/2" (mistake in setting the tumbler for flash hider threads), but I will probably cut it down to 16 1/4" for looks. The midlength looks good with either the full lenth A2 buttstock or the collapsible.

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