BASICS: The dimension of the AR15 buffer tube

An AR15 system has a so-called "buffer tube" in the rear area, which contains the bolt carrier and a buffer. Without this construction, the bolt carrier would not return to the forward position. The stock is also attached to this "buffer tube," which is why this distinctive tube is also often called a "stock tube." However, the first problems arise when you want to replace the collapsible stock, as two slightly different standards have been established: Mil-Spec and Commercial.

So what is the difference between the two? The Mil-Spec tube is slightly narrower than the commercial tube, and the commercial tube is usually chamfered at the end. The exact outer diameters are:

• Mil-Spec: 1.146" or 2.9108 cm
• Commercial: 1.170" or 2.9718 cm

Therefore, the commercial tube is 0.61 mm thicker than the Mil-Spec tube. This difference is so small that you cannot see it with the naked eye, and you may just wonder why the newly ordered stock simply won't fit on or annoyingly wobbles, depending on the case.

To provide an optical difference, most commercial tubes are provided with a 5-6° chamfered end, while Mil-Spec tubes remain straight. Unfortunately, there are reportedly some non-US manufacturers who do not adhere to the chamfering, and you will have to measure again with a caliper.

The good news is that both types fit into the lower receiver of an AR15 in terms of the mounting thread, but the threads on the Mil-Spec variant are said to be more stable.