Mil-spec vs. Commercial buffer tubes for the AR


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wacki
July 11, 2009, 11:58 PM
What's the difference?

Thanks in advance.

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Maverick223
July 12, 2009, 12:04 AM
The commercial is larger and not interchangable IIRC, there is tons of info about this (from much more knowledgeable people I might add), if you search (I know the search feature is a bit lacking...try this instead http://www.google.com/coop/cse?cx=005288657797907197482:juqp1cxyfxy). :)

SHvar
July 12, 2009, 12:06 AM
The inner diameter is the same, the outer diameter with commercial tubes are thicker, therefore a different diameter.
When you remove the stock from the tube you will either see a flat end on the tube, or an angled end, the angled end is a commercial tube. Some manufacturers use one or the other.
The commercial tube and stock were a mistake in a large contract originally, the out of spec stocks, tubes were sold as "commercial" because they did not fit Mil specs. Since then many manufacturers use either one because of advantages one or the other offers.
When ordering a magpul stock I was told that the commercial tube is stronger because it is the same material, but thicker, makes sense.

Z-Michigan
July 12, 2009, 10:32 AM
The commercial tube and stock were a mistake in a large contract originally, the out of spec stocks, tubes were sold as "commercial" because they did not fit Mil specs.

I've never heard this before. Maybe it's true, but do you have any source besides gunshop talk?

When ordering a magpul stock I was told that the commercial tube is stronger because it is the same material, but thicker, makes sense.

All I've seen is the reverse. The commercial tube is an extrusion, the mil-spec tube is a forging. In general forgings are stronger than extrusions (quality control and the exact alloy make a big difference, so don't carry this concept too far). Part of the reason that the commercial is thicker is, I suspect, that the extrusion process does not work for really thin parts. Commercial tubes are cheaper to make and that's why they are more common on less expensive rifles.

Either tube is fine, but mil-spec is better if you're building a rifle.

Some more technical details are available here:
http://www.magpul.com/tech/ctr.html

MrCleanOK
July 12, 2009, 11:39 AM
Commercial tubes have cut threads, milspec tubes have rolled threads. This is why commercial tubes are thicker, folks. To get the same thread dimensions with cut threads, you have to have more material to start with. Rolled threads are stronger (increase in material's yield strength through plastic deformation), and have better fatigue life (sharp corners on cut threads serve as stress concentration points).

Walkalong
July 12, 2009, 12:34 PM
Article with diameters (http://www.ar15pro.net/2008/09/milspec-buffer-tubes-vs-commercial.html)

Arfcom thread (http://www.ar15.com/forums/topic.html?b=3&f=12&t=442122)

Cal Guns thread- nice pics too (http://www.calguns.net/calgunforum/showthread.php?t=153618)

RichTheMac
October 28, 2010, 04:24 PM
There is a $1 little gage for sale on Amazon that lets you tell which kind of tube you have. The gage is made by Tactical Intent, a company that is making buttstocks and rail covers. The gage is dirt easy, cheap and eliminates any question.

You can find it at:
http://www.amazon.com/gp/browse.html?ie=UTF8&marketplaceID=ATVPDKIKX0DER&me=A36ZAOEPX7I0P8

taliv
October 28, 2010, 04:31 PM
or for somewhat less than $1 you could just look at the slanted end

LiquidTension
October 28, 2010, 04:37 PM
Oh that ^ is pure win right there.

Onmilo
October 29, 2010, 09:44 AM
DSA offers a tube that is neither fish or foul.
It is dimensioned between the commercial and mil-spec range and designed to take up slop from loose mil-spec buttstocks and it is also flat on the back.'
How's that for adding even more confusion.

Mil-spec dimension tubes are usually best quality.
I like the VLTOR offerings and use them exclusively now.

Z-Michigan
October 29, 2010, 10:21 AM
I have a DSA 6-position "mil spec" tube in use, and a couple more waiting for use... they are advertised as being mil spec, but I haven't measured them. They work fine with Magpul MOE mil-spec size stocks. I hadn't heard the claim of being an in-between size before.

Any idea how they are made? Half my reason for purchasing a "mil spec" tube was to get the superior forging and threading process used on true mil-spec.

kwelz
October 29, 2010, 10:24 AM
Yep. Usually with "mil-spec" tube you know what you are getting. Commercial, not so much.

Onmilo
October 29, 2010, 01:34 PM
Brownells can explain the DSA tube better than I can.
http://www.brownells.com/.aspx/pid=27440/Product/CAR_15_M4_RECEIVER_EXTENSION_TUBE

Note the, "Machined from a lightweight aluminum extrusion,,,"
Note the flat back.

Not all extension tubes are created alike.

Z-Michigan
October 29, 2010, 01:57 PM
To quote a bit more (and mindful of copyright issues):

M4-style receiver extension tube *** Outside diameter of tube is machined approximately .003" oversized for snug, rattle-free fit of stocks designed for mil-spec extensions. Lightly sand interior of stock to get just the right combination of snugness and freedom over movement. Machined from a lightweight aluminum extrusion before hardcoat anodizing with a wear-resistant matte black finish.

Well that's very interesting. I wondered why the DSA was a good price, and conversely why BCM wanted so much for their mil-spec tubes that I thought were only as good.

It would seem that the DSA tube gets you a good fit with mil-size stocks, and it probably has the threads rolled in with deeper thread depth than commercial spec tubes, but I would ask for a machinist or someone with more knowledge than I have to inspect one and see.

However, as an extrusion it isn't the forging of a true mil-spec tube, so it's probably not as strong. How much that really matters I don't know, but I do know that I would now go with BCM, LMT or another top tier if I were doing a high-end build.

FlyinBryan
October 29, 2010, 02:06 PM
so if my bushmaster rifles are commercial they will measure 1.17?

the mil spec ones might be better, but i dont plan on changing mine if they are commercial.

my needs really dont justify a change. just range shooting and sunday plinking out in the country.

maybe slightly off topic, but ive considered changing my 6 position stocks to the a2 stocks. i had a colt with the a2 stock, and really liked the clean lines. will i need to change the tubes to do that? if i decide to

Z-Michigan
October 29, 2010, 02:17 PM
Yes, a commercial tube is 1.17" and Bushmaster normally/always uses them.

If you want an A2 stock you will have to replace a number of components:
-rifle length buffer tube
-A2 stock itself (plastic shell with buttplate)
-stock screw (connects shell to buffer tube)
-buffer (requires a rifle buffer)
-spring (rifle spring, which is longer and more easily compressed than a carbine spring)

FlyinBryan
October 29, 2010, 02:39 PM
lol, ya, the 6 position is good enough then.

DBR
October 30, 2010, 01:51 AM
Commercial diameter tubes are usually extrusions made of 6061 aluminum. They have cut threads which are usually only 70% +- engagement with the receiver. They may or may not be hardcoat anodized. The end is usually welded on.

True milspec tubes are "impact extruded" which is a form of forging. They are made of 7075 Aluminum which is about 50% stronger than 6061. The end is integral with the body. They have fully formed rolled threads that properly engage the receiver threads. They are hardcoat anodized.

These days from what I can determine some commercially available tubes claiming to be "milspec" are actually machined from 7075 bar stock and have cut threads. I think the Vltor falls into this category. While not as strong as impact extrusions they are still a lot stronger than 6061 extrusions and have correct full depth threads.

I use and have been satisfied with either BCM or Vltor tubes on several recent builds.

Onmilo
October 30, 2010, 08:56 AM
DBR, I believe that is about the best assessment I have seen here.
Properly machined barstock can be just as strong as an impact extrusion.
Thread engagement is dependent on the skill of the machine operator and his ability to set the depth of cut.

I had a 6061 Commercial extension tube fail when I knocked a carbine off of a bench and it fell butt first at angle onto a concrete pad.

The commercial stock latch was broken and the tube bent to an extent the rifle was no longer functional.
The cut threads held and no damage occurred to the receiver.
Both stock and tube were from one major manufacturer and I won't name them because it doesn't matter who, the matter is in what happened with what equipment and other manufacturers use the same set up, mainly in the interest of cutting costs to increase profitability.

Since they could use a higher grade of material in the construction of the rifles and at the current retail pricing of their rifles they could do so without adding downstream costs to the consumer,,, lets just say buying a factory made M16 type rifle is a caveat emptor proposal and the same is true for many if not most bolt, lever, and semi auto sporting rifles offered on the market so it does no good to name names.

At that point my rifle failed I immediately converted to Magpul CTR stocks with the secondary lock and VLTOR tubes.
Since I haven't dropped or abused a rifle since the last incident, I can't say for sure that the tube and stock will survive but my confidence level is high that they will.
I will name names here because this stuff is just plain well made and worth the money spent.

I also know for a fact what will fail when you least expect it to and do not purchase these items any longer.

wally
October 30, 2010, 08:16 PM
Is it just me thinking that worrying about the strength of an AR butt stock is kind of pointless? I mean is there a worse rifle to be carrying if hand-to-hand combat with butt strokes is on the agenda? I'd much rather the stock tube break instead of the the lower if I fall on the rifle.

Z-Michigan
October 30, 2010, 09:27 PM
Wally - no. Commercial spec tubes can sometimes fail with relatively minor hits, like what you might make when going from a sprint to diving for cover. A mil-spec tube is significantly more resistant to such damage, and is pretty well combat proven at this point. And I've never heard of anyone breaking a lower receiver. Sure, the M4 style is pretty bad for hand to hand combat moves, but that's not the only reason to care about the strength of the stock.

FlyinBryan
October 30, 2010, 10:36 PM
has anyone ever heard of a commercial tube actually breaking?

with a link to such maybe?

just curious as ive never heard of a documented case of either type failing. wouldnt it be really rare?

Z-Michigan
October 30, 2010, 10:40 PM
I have heard of commercial tubes failing several times at carbine classes, but I do not have any links, nor have I seen it firsthand. A search of M4c might be useful, or just decide whether spending $30 extra on this part makes sense, or not, for your personal uses.

DBR
October 30, 2010, 10:45 PM
Two other minor points re my previous post:

1) The reason commercial tubes don't have full engagement threads is the major diameter of the thread is limited by the diameter of the extrusion. The thread area is usually the same diameter as the OD of the rest of the tube and is not large enough for full threads.

2) While the material can be the same strength between a 7075 machined bar stock tube and a forged one, the forging should be more resistant to cracking because of continuous grain structure. Rolled threads are better than cut threads for the same reason.

FlyinBryan
October 30, 2010, 10:50 PM
ya Z, i aint asking for the link cuz i dont believe it, lol, i guess that looked bad.

i'd like to see how they break. like where they are threaded (makes sense to me that if one were to snap, it would be at the point of highest stress and at the weakest point)

i guess ive never heard of one breaking, but i have seen a pic (here on thr some time back) where a lower busted where the buffer was screwed in.

taliv
October 30, 2010, 11:08 PM
i can't recall ever hearing of one break, but my memory isn't what it used to be.

FlyinBryan
October 30, 2010, 11:18 PM
do you remember the thread where the lower snapped right by the buffer? or was it the buffer that snapped right by the lower?

ive tried searching for it and cant find it but now im wondering if it was in fact the tube that broke? (my memory is prolly as bad or worse, lol)

it was a couple years back, posted here.

taliv
October 30, 2010, 11:27 PM
i remember a couple discussions about hesse/vulcan lowers breaking, and a couple about olympic too
http://www.thehighroad.org/showthread.php?t=300290

but not an extension

FlyinBryan
October 30, 2010, 11:30 PM
thats it!!! (post 19)

(but i was hoping it was the tube i remembered being busted like cheap plastic. it would have been a great addition to this thread)

Onmilo
October 30, 2010, 11:43 PM
Mine didn't break, it bent.
Doesn't really matter because it prevented the buffer from cycling in the tube and that locked the action up.

I am beginning to photo document the fails and gunsmith projects now.
Something I never did before.

Doubting Thomas still needs to stick his fingers in the wounds.

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