Will a mil-spec AR-15 stock fit a commercial buffer tube?

The two tube types look identical on a shelf and are not. A mil-spec receiver extension measures roughly 1.148 inches across the body; a commercial one is roughly 1.170. That 0.022-inch gap is invisible by eye and decisive in practice.

It matters in both directions, differently. A mil-spec stock will not go onto a commercial tube — the bore is simply too small, and forcing it is how people crack a stock or gall a tube. A commercial stock will go onto a mil-spec tube, but it sits loose and rattles, because it is sized for something fatter.

To identify what you have without a caliper, look at the back end of the tube. A commercial extension has a sloped, angled rear face. A mil-spec one is flat. Stock makers label their products for one or the other, which is the simplest confirmation that the distinction is real rather than marketing.