Sunday, March 23, 2008

mil spec and commercial spec receiver extensions (buffer tubes)

Since I looked for the answer to this question for a very long time, I suspect that I'm not alone. This information was given to me by Pat Rogers. I don't know if these are his words or if he got them from somewhere else, but I owe him a debt of gratitude for providing me with this. I have shared it with a couple of other people, and they had the same "wow, thanks!" reaction that I did. That being the case, I decided to post it up here so that the other two people that read my blog can have that info, too. Thanks, Pat!

"Buffer Tube Basics

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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 1.150, the thread diameter is 1.1875

So the threads are bigger, are they rolled in? The thread are 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.

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. Nearly all of the tubes that I have seen fail in the field broke at the threads or pulled the threads out.

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."

12 comments:

DirtCrashr said...

Mine fit - I bought the whole kit (CTR stock). :-)

Haji said...

Even though the outlay of cash is a bit more up front if you have a comm spec tube, because its gotta be replaced with a mil spec one, the advantages are such that its a small price to pay in my mind.

I'm off to dirtcrashr's blog to see if he posted pics yet!

DirtCrashr said...

I figure just go Mil-Spec all the way!

Doctor Blase' said...

Thanks a bunch for this post, I had spent an hour trying various Google search terms and reading through Forum threads before finding this.

Haji said...

Glad to be of help! Thanks to Pat Rogers for passing this on to me. He's a good man.

Anonymous said...

Nice dispatch and this enter helped me alot in my college assignement. Say thank you you for your information.

greg said...

"wow, thanks!"

lol... i've spent the last 2 days trying to find this kind of clarity of information in one reading. thank you for placing it all together & presenting the larger picture.

Haji said...

After 40+ years using the M16 FOW, The Fighting Irishman has learned a couple things about 'em.;)

I'm constantly humbled by how much Pat knows, and can convey with such ease and style. Pat is the man.

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Anonymous said...

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Anonymous said...

I only got my First AR 5 years ago(only because before that I wasn't old enough. Since then, I've got almost a Dozen(11) AR's ranging from the standard 5.56 NATO chambering, 5.56 Wylde Chambering, .22LR, 6.8SPCII, 6.8SPC Noveske mod 1,.300 AAC Blackout,6.5Grendel, 7.62x51 and 7.62x39. My last aquisition was a Magpul Colt LE6920, To the point, EVERY Commercial Buffer Tube I've seen has NO angle at the rear, but the commercial models are angled in the rear. every mil-spec buffer tube(that I've seen) has NO angle at the back, and every commercial DOES have an angle at the rear, My Dad's Company has a HUGE machine shop, so there are micrometers everywhere and I'd heard of this, but it's how I was able to confirm the rear of the buffer Tube being angled or not. BTW, fav. cal. 6.8, with a Barrett Mag, you can shoot 30+1 rounds of 6.8SPC. Since I've yet to purchase a full auto pre-ban AR(that may change monday), I bought a SlideFire Solutions SSAR-15 and along with a Single Point Sling attachment spade that works with the SSAR-15, BTW, I've never purchased any device such as this, for one they never work and 2 there is no aiming, but a friend got one and at 25 meters, with Iron Sights on my third magazine I emptied a 30round mag furiously fast, shot timer said a little over 3 seconds, less than 3.5, at 25 meters 27 out of 30 hit the Human silhouette Target however 3 of the misses hit close, on the paper, just missing the black part, most were within 4 inches of the center, I brought this product up because, I decided to put it on, in my opinion, a top tier manufacturer, my Bushmaster A3M4 Patrolman's Carbine as the permanent home for the SSAR-15,not as good as Colt, but 2nd best American mass produced Gas impingment AR-15,even though, taking it off is as easy as installing, remove adjustable stock, remove grip, make sure you don't lose the spring and replace the grip with included block, install Slidefire Stock. What does this have to do with buffer tubes? when I got it, I used the commercial spec buffer tube, they recommend a mil-spec tube, they sell one, but say any mil-spec will do, my point is in my case the fit felt better with the original BM tube, so, I put it back