When shopping for collapsible stocks online… I noticed that some models come in 2 different variations. Mil-Spec and Commercial-Spec. EXAMPLE. So what’s the difference, and why are there 2 types?

EDIT Oct 29th 2012: This post generates a LOT of traffic and great comments / communication.  While there are differences between the two, great question has been raised as to whether or not one is better than the other, or if Mil-spec is any more reliable than others.  I like to consider myself a man of science… and will validate that I have not seen any solid proof showing that one style of buffer tube is any better than the other.  We do have to recognize that there are two sizes of tubes available on the market, and this post is created to illustrate those differences.

The two variations of the Stock exists because of two possible sized buffer tubes, or, lower receiver extenders.  I’m going to refrain from calling it the lower receiver extender simply because I find that it is a less commonly used name.  From what I could gather, the reason 2 types of tube exist is because the Mil-Spec version costs more to manufacture. More details… click the post to continue.

The biggest differences between the two tube types are:  1) The buffer tube diameter itself 2) The diameter of the threads in relation to the diameter of the tube.

Mil-Spec: On the mil-spec buffer tube… the threads at the front are actually a larger diameter than the rest of the tube itself.  In manufacturing, a large tube is used and threads lathed as normal. Next, the rest of the tube must be milled down to the military specified diameter.  I’m guessing this step requires more work / precision… and costs more.

Commercial-Spec:  In manufacturing, you begin with a tube that matches the final tube diameter.  The threads are cut into the front end with a lathe just like any other threaded pipe.  I imagine this to be just like the copper water pipe used in my house.  Cheap & simple.

So when shopping for a stock… make sure to get the same type as your buffer tube.  And make sure to get a buffer tube that is specified by the manufacturer of your lower receiver.  I did read that you can use a commercial buffer tube in a receiver made to Mil-Spec… but I don’t have any experience with that.

(Click Image for full-size version, courtesy of LaRueTactical.com)

Sources:

http://hajisplace.blogspot.com/2008/03/mil-spec-and-commercial-spec-receiver.html