AR15.Com Archives
 Buffer tubes, How do you know?
makattack  [Member]
7/3/2010 9:06:12 PM
I have an AR I built with a Model 1 kit a few years ago. It has a Rock River M4 stock I got with the kit. I'm looking to replace it with a Magpul stock. How can I tell if I have a comercial v.s. milspec buffer tube? Thanks
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Gamma762  [Team Member]
7/3/2010 9:18:55 PM


Look at the visible part of the threads compared to the smooth part of the tube.

If it's really an RRA stock it is commercial size.
makattack  [Member]
7/3/2010 10:27:41 PM
It's stamped with the Rock river logo. Should I go ahead and chang e the buffer tube to milspec also?
FALPhil  [Member]
7/4/2010 7:54:43 AM
Should I go ahead and chang e the buffer tube to milspec also?

Only if you are wanting the Magpul UBR stock. Otherwise, Magpul stocks come in two flavors - one for mil buffer tubes and one for commercial buffer tubes. If your tube is commercial, just buy the Magpul stock you want in the commercial offering. I can pretty much guarantee you that your gun won't perform any better if you change out the buffer tube, and the money you save by not swapping the buffer tube can be spent on ammo.
Primos  [Member]
7/4/2010 11:12:43 AM
Originally Posted By FALPhil:
Should I go ahead and chang e the buffer tube to milspec also?

Only if you are wanting the Magpul UBR stock.


UBR uses neither. Comes with its own special tube.
Unholy  [Member]
7/4/2010 6:30:28 PM
If you are planning on getting any of the magpul stocks that use the friction lock like the acs or ctr, get a new tube.

RRA tubes run small and the friction lock wont lock up on them at all.

Alpha-Romeo3  [Team Member]
7/4/2010 10:11:40 PM
Originally Posted By makattack:

Should I go ahead and change the buffer tube to milspec also?


It's up to you if you want to improve your buffer tube with a structurally stronger mil-spec size type.

I would recommend getting the BCM mil-spec size buffer tube because they're slightly larger in diameter compared to other brands of mil-spec size tubes, all my Magpul CTR, MOE, Vltor Modstock, EMOD, Colt M4, and LMT M4 stocks fits snug with it.


FALPhil  [Member]
7/5/2010 7:21:51 PM
It's up to you if you want to improve your buffer tube with a structurally stronger mil-spec size type.

Milspec tubes are not necessarily structurally stronger. Carbine receiver extensions (buffer tubes) have a slightly smaller O.D.(1.14" vs 1.17") than commercial receiver extensions. All other things equal, this means that the commercial buffer tube is conceivably stronger. However, things are not always equal, such as anodizing, forming, material, etc. Now, milspec buffer tubes are forged. I don't know of a single commercial manufacturer that forges their buffer tubes, but I do know of at least two that use DOM tubing, which has some effect at strengthening the tube.

Which one is stronger? Who knows? I have never seen anything on AR15.com or m4carbine.com that led me to believe that anyone on either board can tell you what the failure point is on any manufacturer's receiver extension. I suspect the choice of receiver extension is a tempest in a teapot (like many of the milspec vs commercial arguments that occur on both boards).

You want maximum strength, get a steel receiver extension. They exist. I have seen 2 on privately held M16s.
GHPorter  [Team Member]
7/5/2010 8:40:15 PM
Originally Posted By FALPhil:
It's up to you if you want to improve your buffer tube with a structurally stronger mil-spec size type.

Milspec tubes are not necessarily structurally stronger. Carbine receiver extensions (buffer tubes) have a slightly smaller O.D.(1.14" vs 1.17") than commercial receiver extensions. All other things equal, this means that the commercial buffer tube is conceivably stronger. However, things are not always equal, such as anodizing, forming, material, etc. Now, milspec buffer tubes are forged. I don't know of a single commercial manufacturer that forges their buffer tubes, but I do know of at least two that use DOM tubing, which has some effect at strengthening the tube.

Which one is stronger? Who knows? I have never seen anything on AR15.com or m4carbine.com that led me to believe that anyone on either board can tell you what the failure point is on any manufacturer's receiver extension. I suspect the choice of receiver extension is a tempest in a teapot (like many of the milspec vs commercial arguments that occur on both boards).

You want maximum strength, get a steel receiver extension. They exist. I have seen 2 on privately held M16s.
Commercial tubes are made by drawing, while actually MIL-SPEC receiver extensions are made by a forging process. Drawing pulls on the metal, which can make it fit the size it needs to be, but at the same time it weakens the basic material. Forging hammers the material into shape, which aligns the crystals within the metal and makes it much, much stronger. Commercial tubes have cut threads, as well, which makes them weaker, while MIL-SPEC tubes have rolled threads (another forging-like process), which makes them more robust.

Frankly "all other things" are not equal, as long as you're talking about actually MIL-SPEC versus strictly commercial receiver extensions.

Gamma762  [Team Member]
7/5/2010 8:51:34 PM
Both commercial size and milspec size tubes are made by extrusion which is a linear forging process. Of quality tubes of either size, neither has a significant strength advantage.
FALPhil  [Member]
7/5/2010 9:17:47 PM
Drawing pulls on the metal, which can make it fit the size it needs to be, but at the same time it weakens the basic material.

That's true for the garage mechanic. But in a sophisticated manufacturing process, drawing aluminum will not weaken the metal. Remember, aluminum fails from stretching, not from compression. So, you manage the draw ratio. This is done by maintaining the exterior surface areas and precise position management of the mandrel in the drawing process along with adequate lubrication. Given that the blank is extruded (like all structural aluminum used in everything from skyscraper beams to bicycle axles), precision drawing does not diminish strength. Add to that the fact that the hard anodizing shell also acts as a strength element, and I daresay that there is no discernible d8fference between the two.

Now, what is stronger on a milspec buffer tube is the thread engagement. If you look carefully, you will notice that commercial threads have blunted tops. This is because when they are cut, they are cut with a standard 1-3/16-16-UN (1.1875") thread on the thick(1.17") tube. Milspec threads are cut on a thicker collar which gives them the full 1.1875" diameter with well defined peaks. So, in that sense, the milspec tube is stronger. Is it stronger in any practical sense? The only way to find out would be to destroy either the receiver or the receiver extension or both. I don't know of any tests where this has been done. Either way, the "topped" threads of a commercial tube provide more than enough strength for the intended purpose; they are unlikely to fail in even the most harsh conditions, given the pitch and the number of engaged threads.
Alpha-Romeo3  [Team Member]
7/5/2010 10:11:50 PM
Originally Posted By FALPhil:
It's up to you if you want to improve your buffer tube with a structurally stronger mil-spec size type.

Milspec tubes are not necessarily structurally stronger. Carbine receiver extensions (buffer tubes) have a slightly smaller O.D.(1.14" vs 1.17") than commercial receiver extensions. All other things equal, this means that the commercial buffer tube is conceivably stronger. However, things are not always equal, such as anodizing, forming, material, etc. Now, milspec buffer tubes are forged. I don't know of a single commercial manufacturer that forges their buffer tubes, but I do know of at least two that use DOM tubing, which has some effect at strengthening the tube.

Which one is stronger? Who knows? I have never seen anything on AR15.com or m4carbine.com that led me to believe that anyone on either board can tell you what the failure point is on any manufacturer's receiver extension. I suspect the choice of receiver extension is a tempest in a teapot (like many of the milspec vs commercial arguments that occur on both boards).

You want maximum strength, get a steel receiver extension. They exist. I have seen 2 on privately held M16s.

The persons that would really know are the military and the manufacturers, and the military specified the forged 7075-T6 material which is in their TDP/ Mil-Spec which they know is a stronger material than non forged aluminum of the same material type.

Even if we don't know the actual catastrophic failure point of forged and non forged aluminum tubes, we could just look at the difference in the strength of materials which would state that the forged material is stronger given the same thickness, even if you add 0.015" more material thickness to the weaker material I don't believe it will even equal the stronger forged material.

Another difference between the mil-spec and commercial buffer tubes are how the threads are made, the mil-spec threads are rolled while the commercial threads are machine cut, the rolling process is stronger because it kind of cold works the material, similar to cold forging.

From the many discussions here in the past the most common area that breaks in carbine receiver extensions are either at the threaded or near the threaded areas.

Using steel RE on semi auto AR is an overkill IMHO.


TigerForce  [Team Member]
7/6/2010 1:05:13 AM
I read somewhere here that Colt and LMT make their extension tubes about as close to "mil-spec" (sorry for using this "overused" term) as any available on the civilian market. including dry film coating. However, I don't know the details about the manufacturing process other than that. I have an LMT extension that came on a Defender 2000 lower and it appears to have the rolled threads, and does have the dry film coating, which is kind of a dark grey color.

ETA: And it looks like no one has posted this link yet, from Magpul.
GHPorter  [Team Member]
7/6/2010 7:21:12 AM
Originally Posted By FALPhil:
Drawing pulls on the metal, which can make it fit the size it needs to be, but at the same time it weakens the basic material.

That's true for the garage mechanic. But in a sophisticated manufacturing process, drawing aluminum will not weaken the metal. Remember, aluminum fails from stretching, not from compression. So, you manage the draw ratio. This is done by maintaining the exterior surface areas and precise position management of the mandrel in the drawing process along with adequate lubrication. Given that the blank is extruded (like all structural aluminum used in everything from skyscraper beams to bicycle axles), precision drawing does not diminish strength. Add to that the fact that the hard anodizing shell also acts as a strength element, and I daresay that there is no discernible d8fference between the two.

Now, what is stronger on a milspec buffer tube is the thread engagement. If you look carefully, you will notice that commercial threads have blunted tops. This is because when they are cut, they are cut with a standard 1-3/16-16-UN (1.1875") thread on the thick(1.17") tube. Milspec threads are cut on a thicker collar which gives them the full 1.1875" diameter with well defined peaks. So, in that sense, the milspec tube is stronger. Is it stronger in any practical sense? The only way to find out would be to destroy either the receiver or the receiver extension or both. I don't know of any tests where this has been done. Either way, the "topped" threads of a commercial tube provide more than enough strength for the intended purpose; they are unlikely to fail in even the most harsh conditions, given the pitch and the number of engaged threads.
Drawing is drawing, and it is inferior to forging in terms of overall strength. Even if a draw process does not diminish strength, it doesn't enhance it, as the forging processed used in making truly MIL-SPEC extensions do. Further, MIL-SPEC receiver extension threads are ROLLED, not cut. Thus the specific profile of the threads is different, and the threaded end is thus much stronger.

FALPhil  [Member]
7/6/2010 8:51:51 AM
Thus the specific profile of the threads is different, and the threaded end is thus much stronger.

Only in tensile strength. There is no benefit to rolled threads for shear strength. Rolled threads were chosen, not for strength, but as a concession to manufacturing economy anyway. That's also the reason that 7075T6 was chosen as a material - because the properties of that alloy fit the minimum elongation factor of 5 percent and the maximum tensile yield strength for rolling threads. I mentioned the profile in my previous post and the fact that the number and pitch of the threads means that there is no practical difference in strength, that is, the metal will fail before the threads fail.

From the many discussions here in the past the most common area that breaks in carbine receiver extensions are either at the threaded or near the threaded areas.

I hear you about the failures. The problem is, the reports we hear never include a laboratory analysis of the failure. In fact, they don't even include hardly a cursory analysis of the failure. Most of the analysis consists of conjecture and assumption. So the cause of failure could be a substandard material, or a substandard anodizing process, or shoddy machining - we just don't know. But then, even milspec receiver extensions fail, too. Plus, there are vendors out there with "milspec" receiver extensions which are in actuality no such thing. So, I think the moral of the story is, if you buy a buffer tube from a reputable manufacturer of known quality, it doesn't really matter if it is "milspec" or commercial. If you go another route, you are taking a greater risk.
GHPorter  [Team Member]
7/6/2010 8:35:26 PM
Originally Posted By FALPhil:
I mentioned the profile in my previous post and the fact that the number and pitch of the threads means that there is no practical difference in strength, that is, the metal will fail before the threads fail.
Being a "belt and suspenders" kind of guy, I'll stick with the stronger structure, even knowing that the metal is the weak link in the part. But I do understand where you're coming from...

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