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Thread: Buffer Tube Basics

  1. #1
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    Buffer Tube Basics

    I know, the correct name is receiver extension tube... so now that we got one of most common mistakes out of the way, let's look at some common questions about receiver extension tubes. I do not pretend to know everything about these rifles, so if you see anything that is wrong, let me know.

    This info is for carbine/short/collapsable tubes - all rifle/long/standard tubes are the same (as far as dimensions go)

    What is a Mil-Spec Tube? This has to be the most common question. A milspec tube is just as the name implies, made to the specifications of the technical data package (tdp). The biggest concern is the diameter of the tube, the milspec part has a diameter of ~1.147, the thread diameter is 1.1875

    So the threads are bigger, are they rolled in? originally the threads were cut in with a regular old lathe. The narrower body section of the tube is reduced by cutting with a concave shaped cutter that "shaves" the metal down, sort of like a plane; occasionally you can see the result of this technique by two "lines" that run the length of the reduced section at ten and two o'clock. Current techniques do include rolling the threads in.

    So what are tubes that are not Mil-Spec? During the original cloning of the military AR, the first commercial receiver extension tubes were made from an extrusion, the threads lathe cut and the body was not reduced. The common size for this is 1.170 and that has became the de facto standard for after market tubes.

    So commercial tubes are bigger, are they stronger? No, the problem with them is that the threaded section is also 1.170, so the threads are not cut to full "height" and do not fully engage the threads in the receiver. The few commercial tubes that I have seen fail, pulled the threads out of the receiver.

    Are there any other concerns with after market tubes? Well, there is really no set standard for size, so they can vary by manufacturer and even from lot to lot -- you can get combinations of after market parts that are tight or loose. Some of them are made with extrusions with an end plug welded in, the quality of the welding can run from pretty solid to pretty poor.

    Are those the only two sizes? No, some manufacturers have come out with systems that do not use either the milspec or the aftermarket tube size... The early Choate stock is one example of a proprietory tube.

    So what fits what? Milspec tubes work with stocks from Colt, Vltor, CMT, LMT/Crane, etc.

    After market tubes work with RRA, BM, DPMS, etc.

    A larger, after market stock will fit on a milspec tube, but will be loose and rattle -- a milsec stock will usually not fit on an after market tube... without a hammer.

    Hey! My new Mil-Spec tube is too long? Some newer tubes made for the "enhanced" M4 stock are a little longer and have a small taper to the back to match the profile of the stock butt. It should not effect the use of a Mil-Spec stock however.

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    Buffer Tube

    And that is why when I tried putting a LMT stock on my RRA tube a couple years ago I ended up changing the tube because I didn’t have the heart to use the hammer you mentioned. Thanks, that explained a lot.

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    so all the threads are the same size? can any buffer tube locking nut fit any buffer tube? Must a buffer tube be staked to stay in place(will those new Ace thin lock rings work)?

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    Stickman, any chance you could reload those pics? I'm getting the dreaded "red x's". Thanks.
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    Quote Originally Posted by FlyAndFight
    Stickman, any chance you could reload those pics? I'm getting the dreaded "red x's". Thanks.
    Yep, me too.

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    Some newer tubes made for the "enhanced" M4 stock are a little longer and have a small taper to the back to match the profile of the stock butt. It should not effect the use of a Mil-Spec stock however.

    Should I be confused?

    I called Magpul a week or so ago and the rep I was on the phone with was describing to me how to ID my tube if it were mil spec or not, this was in an effort to see if my tubes would work with a CTR.

    The tubes I have do NOT extend past the buttstock but they are slightly canted to follow the form of the buttstock.

    I was told the canted tube means it is NOT milspec.

    If I read this thread right, either I heard wrong or the Magpul guy is wrong.

    I am willing to bet a kick between the legs that I heard wrong.
    “For those who have fought for it, freedom has a taste the protected will never know.”

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    Is it possible to buy mil-spec tubes?
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    Quote Originally Posted by Batt 57
    Is it possible to buy mil-spec tubes?
    Yes, they're available from VLTOR, LMT, CMT.
    www.virginiaarms.com
    robb@virginiaarms.com

    Virginia Arms Co FFL/SOT Armorer
    Corporate Arms FFL/SOT type 07 & 08 Armorer

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    Quote Originally Posted by Chris_C
    Some newer tubes made for the "enhanced" M4 stock are a little longer and have a small taper to the back to match the profile of the stock butt. It should not effect the use of a Mil-Spec stock however.

    Should I be confused?

    I called Magpul a week or so ago and the rep I was on the phone with was describing to me how to ID my tube if it were mil spec or not, this was in an effort to see if my tubes would work with a CTR.

    The tubes I have do NOT extend past the buttstock but they are slightly canted to follow the form of the buttstock.

    I was told the canted tube means it is NOT milspec.

    If I read this thread right, either I heard wrong or the Magpul guy is wrong.

    I am willing to bet a kick between the legs that I heard wrong.
    I am sure it must be me that is wrong...

    Manufacturers can make whatever they want, but the easy way to tell (other than actually measuring) is to look at the area where the key for the locking holes transitions into the buffer body... a milspec tube will have a sharp, very small radiused corner that looks almost square -- while the aftermarket tubes have a much larger radius, making a fillet at the corner.
    The sun will rise tomorrow; it’s your attendance of the event that is uncertain.

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  10. #10
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    Quote Originally Posted by FlyAndFight
    Stickman, any chance you could reload those pics? I'm getting the dreaded "red x's". Thanks.
    Sorry about that, they are back up now.
    Stick


    I have been asked to point out by board policy that I am Magpul's photographer. I have also done work for 150 or so manufacturers within the firearm community, my work has been used for web, catalog, banners, magazines, covers, etc. I have also shot work for the US Military where it has been used in training aids. I am prior service and a full time LEO.

    I also shoot and write for various publications. My primary focus is weapons, accessories and related equipment reviews. Manufacturers can reach me through any of the boards. If you have something new, let me know.

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