Short answer: a mil-spec buffer tube is about 1.148 inches across; a commercial tube is about 1.168 inches. Collapsible stocks are made for one or the other: a mil-spec stock won't slide onto a commercial tube, and a commercial stock wobbles on a mil-spec tube. Most aftermarket stocks are mil-spec, so for a new build buy a mil-spec buffer tube.
Mil-spec vs commercial at a glance
| Mil-spec | Commercial | |
|---|---|---|
| Outside diameter | About 1.148 in | About 1.168 in |
| Rear of the tube | Flat | Usually sloped (angled) |
| Threads | Usually rolled | Usually cut |
| Typical material | 7075-T6 aluminum | Often 6061 aluminum |
| Stocks that fit | Most aftermarket stocks | Stocks sold as "commercial" |
| Buffer and spring | Standard carbine buffer and spring | The same buffer and spring |
How to tell which one you have
- Measure it: calipers across the tube read about 1.148 in for mil-spec, 1.168 in for commercial.
- Look at the back: a flat end is usually mil-spec; a sloped, angled end is usually commercial.
- Check the listing: most tubes and stocks say "mil-spec" or "commercial" in the name.
Only the diameter decides which stock fits. The buffer and action spring inside are the same for both, so a carbine buffer works in either. Rifle-length and A5 tubes are a different length, with their own buffers and springs.
Parts that fit
Compare mil-spec buffer tubes, all buffer tubes, buffers and springs and AR-15 stocks by price. Add them to the AR-15 builder and it flags a stock that doesn't match your tube.
Building an AR pistol? See our AR pistol build guide for pistol buffer tubes and braces.
Know your tube? Compare stocks that fit it in our best AR-15 stocks guide.
Choosing between a fixed and a collapsible stock? Read our fixed vs collapsible AR-15 stocks comparison.
For the spring that goes inside, see AR-15 buffer springs.
Frequently asked questions
Will a mil-spec stock fit a commercial buffer tube?
No. The commercial tube is slightly fatter, so a mil-spec stock won't slide on. A commercial stock on a mil-spec tube fits but wobbles.
Is mil-spec better than commercial?
Mil-spec tubes are usually 7075-T6 aluminum with rolled threads and fit far more stocks, so they're the better choice for a new build. A commercial tube works fine with a matching stock.
Do mil-spec and commercial tubes use different buffers?
No. A carbine buffer and spring work in either; only the outside diameter differs.
How do I measure my buffer tube?
Measure across the tube with calipers, away from the threads: about 1.148 in is mil-spec and about 1.168 in is commercial.
