Short answer: the two tubes differ slightly in outer diameter, so stocks made for one may not fit the other. Match the stock to the tube before you buy.
Why it matters
Mil-spec tubes are a little smaller across than commercial tubes. A stock made for one can be loose or too tight on the other.
How to tell
Measure the outside diameter or check the manufacturer listing. When unsure, buy the stock and tube as a set.
Related parts
The tube also pairs with the buffer and spring. See the buffer system guide.
Why the difference matters
The buffer tube, or receiver extension, is the tube that threads into the back of the lower receiver and holds the buffer and spring. Stocks slide over the tube, so a stock made for one size will not fit the other. Buying the wrong stock for your tube is one of the most common AR-15 build mistakes.
The two sizes
Mil-spec tubes are slightly smaller in diameter, around 1.148 inches on the outside. Commercial tubes are slightly larger, around 1.17 inches. Mil-spec tubes are usually made from 7075-T6 aluminum and follow the military drawing. Commercial tubes follow a looser standard, and dimensions can vary more between makers.
How to tell which one you have
- Measure the outside diameter of the tube with calipers. About 1.148 inches means mil-spec, and about 1.17 inches means commercial.
- Check the markings on the tube or the stock listing, which often says mil-spec or commercial.
- Try a stock that is sold for the tube type you think you have. If it wobbles or will not slide on, the sizes do not match.
What else to match
- The stock must match the tube type, mil-spec or commercial.
- The buffer weight and spring must match the tube length and the rest of the gas system. See our AR-15 buffer weight guide.
- Use the right castle nut and end plate for the tube, and stake the castle nut so it cannot back out.
Which to pick
Mil-spec is the safer choice if you want the widest selection of stocks and a tight fit with less wobble. Commercial tubes work well with stocks made for them. If you are unsure, buy the buffer tube and stock together as a kit so they are matched.
Keep building
Browse AR-15 parts, read the custom AR-15 build guide, or let our Palmetto, FL shop help with gunsmithing and custom AR builds.
Shop AR-15 parts at BMTC USA, a gun shop and gunsmith in Palmetto, FL.