Mil Spec vs Commercial Buffer Tube: How an AR-15 Stock Mounts, in 3D
See an AR-15 buffer tube, end plate, castle nut and stock go on in 3D, walk the stock through its six positions, and put a mil spec and a commercial tube side by side to see why a stock only fits one of them.
Quick answer
- An AR-15's stock rides on the receiver extension, the buffer tube, which screws into the back of the lower and is locked by an end plate and a castle nut.
- Mil spec and commercial carbine tubes differ by two hundredths of an inch: 1.148 inches against 1.168. A stock only fits the tube it was made for.
- Almost every stock sold today is mil spec. Both tubes take the same carbine buffers and springs and fit any AR-15 lower.
Source: maker specifications and the GunPartPicker catalog.
See how a stock goes on, in 3D
Watch the buffer tube, end plate, castle nut and stock go on, walk the stock through its positions, or put a mil spec and a commercial tube side by side. Step through it, scrub it, or drag to turn it.
How it goes on · 1 of 6Thread the tube into the lower
How it goes on · step 1 of 6
Thread the tube into the lower
The receiver extension, the part everyone calls the buffer tube, screws into the threaded ring at the back of the lower. Its index rail goes under, lined up with the stock's lock.
Illustrative, not to scale: the parts are drawn shapes, and the compare view draws the 0.02 inch difference six times larger so you can see it.
| Mil spec | Commercial | |
|---|---|---|
| Outside diameter | 1.148 in | 1.168 in |
| Thread into the lower | 1 3/16 x 16, rolled | 1 3/16 x 16, cut |
| Rear of the tube | Flat, square to the tube | Sloped |
| Stocks that fit | Mil spec stocks only: nearly every stock sold today | Commercial stocks only; a mil spec stock won't slide on |
| Buffers and springs | Carbine, H, H2, H3 | The same |
| Lowers | Any AR-15 | Any AR-15 |
Mil spec buffer tubes and kits in stock
Ranked by what builders add most, in stock first. One listing per model.
Commercial buffer tubes in stock
Ranked by what builders add most, in stock first. One listing per model.
Carbine stocks builders pick
Ranked by what builders add most, in stock first. One listing per model.
What is the difference between a mil spec and commercial buffer tube?
The outside diameter. A mil spec carbine tube is 1.148 inches across and a commercial one 1.168, and a stock is bored for one or the other. A mil spec stock won't go onto a commercial tube, and a commercial stock rattles on a mil spec one. Mil spec tubes are usually 7075 aluminum with rolled threads; commercial ones are often 6061 with cut threads and a sloped rear.
Both thread into any AR-15 lower and take the same carbine, H, H2 and H3 buffers and the same spring. Pick a buffer for your gas system in the buffer weight calculator, and watch it work in the AR-15 buffer system in 3D.
How do you install an AR-15 buffer tube?
In five steps. Put the buffer retainer and its spring in the lower, then thread the tube in until it just covers the edge of the retainer's head, with its index rail straight down. Slide on the end plate, with the rear takedown pin's detent and spring in it, run the castle nut up and torque it to about 40 ft lb with a castle nut wrench. Stake the end plate into the nut, then slide the stock on.
Pistols take a pistol tube with no index rail, and fixed rifle stocks take a longer rifle tube and rifle buffer and spring. See how the tube adds to a short build's length in the SBR planner, and how the upper comes off in upper vs lower receiver.
Frequently Asked Questions
What is the difference between a mil spec and commercial buffer tube?
The outside diameter: 1.148 inches for mil spec, 1.168 for commercial. A stock is bored for one, so a mil spec stock won't fit a commercial tube and a commercial stock rattles on a mil spec one. Commercial tubes also usually have a sloped rear and cut threads.
How do I tell if my buffer tube is mil spec or commercial?
Measure it with calipers: about 1.148 inches is mil spec and 1.168 is commercial. A commercial tube's rear end is usually sloped, where a mil spec tube's is flat.
Do mil spec and commercial tubes take the same buffer?
Yes. Both carbine tubes take the same carbine, H, H2 and H3 buffers and the same carbine spring. Only the stock fit differs.
Will a mil spec stock fit a commercial buffer tube?
No. The commercial tube is two hundredths of an inch fatter, so a mil spec stock won't slide on or binds partway. Go the other way and a commercial stock slides onto a mil spec tube but wobbles.
How tight should the castle nut be?
About 40 ft lb, set with a castle nut wrench, then staked. Staking punches the end plate's metal into the nut's notches so it can't back off and let the tube and stock turn.
How many positions does an AR-15 stock have?
Six on most carbine tubes. The lock lever's pin drops into one of six holes in the rail under the tube, about two thirds of an inch apart.
Cite this tool: GunPartPicker, “Mil Spec vs Commercial Buffer Tube: How an AR-15 Stock Mounts, in 3D”, https://gunpartpicker.net/tools/mil-spec-vs-commercial-buffer-tube. Data from the GunPartPicker catalog and real community builds, prices checked October 29, 2026. Free to cite with attribution.
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