Various manufacturers for whatever reason ($$) have decided to use a completely different size/spec buffer tube on their rifles. It seems like it would be easy to simply make them to the same specs as everyone else but they don't always.
To the naked eye, or even upon a simple look at the buttstock, it may appear as if their are the exact same. However, the length and diameter of the buffer tubes (as well as the buttstocks themselves) are different. These non milspec buffer tubes are referred to as "Commercial" buffer tubes.
The diameter of the milspec buffer tube is 1.14", they are made from 7075 forged aluminum. The commercial buffer tube diameter is 1.17", made from 6061 extruded aluminum, with the commercial version often being longer and slanted in the rear.
This has become an issue lately with people upgrading their stocks to the fairly new CTR buttstock by Magpul, as the milspec (original) version was too small to fit over the commercial buffer tubes. In response, Magpul also makes a commercial version. However, the commercial version requires the use of the buttpad (making it longer and more expensive), whereas the milspec version does not require that.
The CTR appears to be an EXCELLENT value in comparison to it's competition as it is only about a $90-110 upgraded, depending on where you get it and if you need the commercial or milspec version. For a collapsible stock, it is rock solid.
It's not that the regular 6 position collapsible stock is all that bad, but the CTR is obviously more rugged. It has no play on the receiver extension, it's a very good upgrade for the AR15 carbine. My personal favorite.