AR-15 adjustable buffer tubes (correctly known as ‘receiver extensions’) come in two different sizes. It is important to know which tube you have as this impacts your selection of a buttstock. A mil-spec stock will fit on a commercial tube, but will be wobbly, however a commercial stock will not fit a mil-spec tube at all. Knowing your parts allows you to make the correct choice.
Check out this picture and look at the differences in tubes.
You will first notice the mil-spec tube is slightly larger in diameter. When looking at tubes without a micrometer, it is easier to see that the commercial tube is slanted 5* at the rear when viewed from the size, while a mil-spec tube rear is straight and perpendicular to the side. The other minor difference and perhaps the most important for strength are the threads. On a mil-spec tube the threads are cut correct with full engagement where a commercial tube has extruded threads that are rounded off instead of having sharp peaks.
If your part was a cheaper tube or in a kit chances are it’s a commercial tube. There’s no *huge* difference between tubes except size, so don’t sweat it unless you are butt-stroking tangos often or breaching doors. Me, I want what is closest to what the military is using so I pay the extra couple bucks and go with a mil-spec tube for the piece of mind.