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Mil Spec vs. Commercial Buffer Tubes & Stocks – What’s the Difference?

This is one of the most common questions we are asked, and for good reason. If you are interested in upgrading your AR-15 stock, or planning to build an AR, it is critical to understand the subtle differences between these two very similar standards.

Buffers, Tubes, Stocks, and Springs: Definitions

The AR-15 buffer tube (also known as the receiver extension), is the metal protrusion that sticks out of the rifle and serves two functions: it houses the recoil buffer mechanism, and it serves as the attachment point for the buttstock.
The reason it’s referred to as a “buffer tube” is because inside the tube is a large spring (called the action spring), and a weighted part known as the “recoil buffer”.

More info: When the rifle is “cocked” using the charging handle, the action spring is what makes the bolt slide back forward. Also, after a round is fired, the action spring is what pushed the bolt back forward and chambers the next round. The recoil buffer sits inside of the spring, and its weight provides some “buffer” against the forces of recoil. By changing the weight of the buffer, you can change the characteristics of how the rifle cycles.

Difference between Mil Spec and Commercial Buffer Tubes

Now that you know what’s going on inside the tube, let’s examine the difference in types. The most critical difference is the diameter of the area where the stock slides onto the tube. The mil spec tube is about two hundredths of an inch smaller than commercial. This may not sound like much, but it’s enough that buttstocks designed for one standard will not be compatible with the other.

There are other differences that people will point out, such as the angle of the back of the tube, or the type of metal used. The problem is – these traits can vary by manufacturer, and are not necessarily related to the diameter of the tube.

More info: The below picture shows the different dimensions of the 2 items

I’m looking to buy a new buttstock. How can I tell which size I need?

As we mentioned above – you need to match the stock to the buffer tube. Most stocks come in both mil-spec and commercial varieties.
The best way to measure is by using a micrometer, and measuring the diameter of the tube. A mil-spec tube should measure about 1.146”, and a commercial will be about 1.17”.

Why do 2 different standards exist?

As with so many things – it all comes down to money. The mil-spec tubes were the original design, but the manufacturing processes of commercial spec buffer tubes provide a lower cost.

More info: When the M4 was first introduced as a military weapon, there was only 1 type of tube – mil spec. But as the M4-pattern rifles moved into the civilian ranks, companies realized that they could make the tube cheaper. By making the tube a touch larger in diameter, the threads could be cut in, vs the mil spec tube where the threads are rolled in.

So which one is better – Mil Spec or Commercial?

First off – we cannot find any scientific evidence supporting that one standard is noticeably “better” than the other on strength, performance, etc. But we recommend using a mil-spec buffer tube.

Why? Even though it costs a little more, there are more options for aftermarket stock upgrades, and many people believe that the part is slightly stronger (again, this lacks the scientific proof we would like to see).

More info: We see increasing movement toward this becoming the “standard”, in spite of the fact that it tends to costs more. No doubt, the fact that the military favors this standard is a big part of this. For instance, Magpul recently reduced the number of commercial spec stocks that it offers, and now only offers most commercial stocks in black.

One Last Tip

If there's anyone that knows the AR-15 platform, it's the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.
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Jerry Caliendo

August 03, 2016

Shouldn’t that buffer tube dimension be 1.146 not 1.46 as stated for mil-spec in the question about buying a new buttstock ? It might just be a typo, but in building an AR accuracy counts.

AT3 Tactical

August 03, 2016

That was a typo – fixed now – Thanks Jerry!


August 15, 2016

Mil-spec buffer tube material: 6061-T6 or 7075-T6. I replaced a so-called mil-spec tube made from 6061-T6 to a 7075-T6. Is this a necessary move? The tensile strength of the 6061 is half of the 7075-T6. And the cost is no more than a 6061 if catch the 7075 when they are on sale. Also, what’s the TRUE story on BCG, the price for some, way too much money? HP/MPI testing.