What’s the difference between a commercial and Mil Spec adjustable stock?
Is it only related to durability?

The size of the buffer tube is the difference between them.

Different diameters, difference in the way the threads are cut, usually angled back on the commercial. Mil-spec tube isn't as thick. A mil-spec stock will not fit on a commercial tube. I'm sure there's a picture around here somewhere.



Added link to pic: Linky Linky!

Great, thanks for the info.
I'm just trying to determine if it is worth the extra money going to Mil Spec?

From the looks of the link it looks like only one type will fit on any given lower?

Either should thread into the same lower.  My understanding is that all lowers use the same thread, but the commercial tubes aren't cut as deep as mil-spec.

You are correct.

While MIL-SPEC receiver extensions are not as thick as commercial tubes, real MIL-SPEC extensions are made with a process that makes them potentially much stronger, and instead of cut threads, their threads are rolled, which is also stronger.  The price difference isn't that much; given the choice, I go MIL-SPEC.

All the higher end/better stocks are for milspec tubes.

Milspec = more stock choices.

Since the other members explained the difference of the two stock sizes I would recommend just getting the mil-spec size buffer tube.

For the stock either the BCM M4 mil-spec size (GEN 2) if you want a more traditional look, for a modern equivalent get the Magpul MOE mil-spec size, and if you don't want any stock movement at all get the Magpul CTR mil-spec size stock, it have a friction lock that virtually eliminate any stock movement plus it have an additional QD sling swivel mount.


http://www.bravocompanyusa.com/M4-Stock-Milspec-p/stock%20m4%20milspec%20black%20-%20gen2.htm

just buy a quality brand name tube, seems like alot of companys make stocks to fit both, i use the commercial tube mine came with and have a real nice vltor stock on it, if I was building a gun i would probly have bought the milspec tube cause it's milspec

Adding to what was already said.

When the Military decided on intorducing the M4 an important consideration was compatibility of the lower receivers. IOW, the M4 receiver extension needed to have the same threads as the M16 receiver extension. The M4 receiver extension was designed ground up and has a fairly tight tolerance on OD as will as tube thickness. Companies like RRA/Bushmaster/ DPMS take advantage of commercially available raw material which is slightly greater in diameter and has a much larger range of OD. This larger range of OD contributes to ill fitting stock bodies as will as less material under the threads which conceivably makes it weaker.

I haven't seen the material specs on either but I can say that my LMT receiver extension is much more robust looking/feeling than the typical Commercial or comercially available "Mil spec" extensions.