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.
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.
