Buffer Tube Basics

AR/M4 AR Technical Discussion
1

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

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

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)?

4

Stickman, any chance you could reload those pics? I’m getting the dreaded “red x’s”. Thanks.

5

Yep, me too.

6

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.

7

Is it possible to buy mil-spec tubes?

8

Yes, they’re available from VLTOR, LMT, CMT.

9

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.

10

Sorry about that, they are back up now.

11

Thanks alot. Pretty much what I expected, but wanted to be sure I wasn’t missing something.

12

What about CAA buttstocks - after market or MILSPEC?

Thanks.

Jim

13

As far as I know their non-milspec and Jim you don’t want one.

14

Hello gentlemen,
On the last post about not wanting a CAA stock??? Why not? Don’t worry I don’t have one. And this as you can see is my first post. But I want to thank everyone for all the info I have read over the hours and hours I’ve visited. THANK YOU.

AL

15

Because for roughly $15-20 more than the CAA you could have a CMT mil-spec stock. I highly recommend mil-spec receiver extension over commercial tubes if this is a rifle that you might depend on to defend yourself. I also recommend staking the castle nut after torquing properly, it may never come loose on it’s own and the staking ensures that it won’t, it’s the same exact reason carrier key screws are staked. Proper torquing and using high quality parts and it may never come loose, staking ensures that it won’t.

The mil-spec tubes have the full/completed threads and won’t spin off with the castle nut, gouging the tube as RRA sometimes do, Bushys usually don’t do it but they’re not mil-spec either. I have found a technique to avoid this with the Loc-Tited on RRA receiver extensions.

16

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.

17

Can we get a list of which manufacturers sell their lowers with Mil-Spec buffer tubes?

18

I just orderd the CTR/and milspec tube combo from Grant. Figgured get it done in one shot. I am just wondering if the hbuffer is good for bolth carbine and mid length rifles?

19

Hey guys,

First post here. I’m wanting to upgrade my RRA with a new CTR and if I’m gonna do that I might as well change the tube to a milspec. Am I correct in assuming the buffers are compatible between the two different sizes as I’ve never seen any buffers marked specifically for commercial or mil-spec? What about the back plate, nut etc? I have a couple of each laying around so I’d rather just pick up the tube and stock body if possible. Plus is there one mil-spec tube thats better than the others? Thanks for the info!

-RD62

20

Most tubes with the cant are comm. tubes. The exception are a few CMMG tubes that are mil spec dems. and have a canted end.

I owe you a kick in the nads;)