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  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.

  2. #2
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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.

  3. #3
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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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    Quote Originally Posted by K.L. Davis View Post
    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.
    I can confirm that the tubes CMMG uses, combined with the Magpul CTR stock are not entirely compatable. They have this taper and longer length you mentioned.

    My CMMG rifle will not allow the CTR to engage at the shortest position.

    I brought this up to them (CMMG) after receiving my rifle and they simply told me that "Yes, the CTR will not lock up on the shortest position with our tubes." Frankly, I would have liked to have been told this prior to having my rifle built, because I would have been happy to ship CMMG the proper tube as to avoid this aggrivation from happening at all.

    There are two solutions. One of which was addressed by Magpul themselves. Magpul sent me a butt pad/spacer for my CTR that is thicker than what is now currently available (the thicker pads should be available soon...I got one shipped early...thanks Magpul!) These pads/spacers have a deeper recess for the tube (maybe a half inch) than the current CTR pads.

    Once installed, this thicker pad/spacer for the CTR just barely allowed the CMMG tube to lock up on the shortest position, but, trying to UNLOCK it from that position took an incredible amount of force on my part, making this an unacceptable solution.

    The other solution is a 5 position mil spec tube available from DSA's web page. It's shorter and I have been told the CTR will function as it should with this tube. I haven't replaced my tube with this one yet, but I will.
    AMERICAN INFIDEL

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    Quote Originally Posted by 5POINT56 View Post
    (snip)...

    The other solution is a 5 position mil spec tube available from DSA's web page. It's shorter and I have been told the CTR will function as it should with this tube. I haven't replaced my tube with this one yet, but I will.
    Can anyone confirm the 5 positions are different in major dimensions from the usual six position? I always assumed they were the same OAL and overall adjustment range just with longer spaces between detents.

    I'm getting ready to order a Milspec tube for my ACS today so this thread being bumped up like this is timely!

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    Quote Originally Posted by HeavyDuty View Post
    Can anyone confirm the 5 positions are different in major dimensions from the usual six position? I always assumed they were the same OAL and overall adjustment range just with longer spaces between detents.

    I'm getting ready to order a Milspec tube for my ACS today so this thread being bumped up like this is timely!
    I just compared a VLTOR 5 position RE to a Stag 6 position RE, and they were the same over all length. The 5 holes are just spaced a little farther apart as you suspected.

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    I would have been surprised if the length was different, but you never know until you ask. Thanks!

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    Pic of staking?

    Does anyone have a photo of the staked castlenut and plate so I can see the index points??

    Wish I had read this 2 weeks ago. I just put a Magpul MOE on my commercical tubed Bushy and now I wish I had swapped the comm. tube for mil spec and gone with the mil spec MOE stock! DOH!

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    Bump...

  10. #10
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    More questions....

    Ok, so where does this put ACE stocks, especially the SOCOM stocks?

    Does this threading issue only apply to extensions or does it also apply to the lower receiver?

    In other words, do all lowers have the correct milspec thread?

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