What Is the Difference Between Mil-Spec and Commercial Buffer Tubes?
Mil-spec buffer tubes have a 1.148-inch outer diameter and use a type III hard-coat anodized aluminum finish. Commercial buffer tubes are slightly larger at 1.168-inch diameter with a smoother finish. Mil-spec is the industry standard — the vast majority of aftermarket stocks are designed for mil-spec tubes. Always buy mil-spec unless you have a specific reason not to.
Why It Matters
A stock designed for a mil-spec tube will wobble on a commercial tube, and a commercial stock may not fit a mil-spec tube at all. Most quality AR-15 manufacturers ship mil-spec buffer tubes, but some budget rifles come with commercial. Knowing which you have determines which stocks and accessories are compatible, and buying the right one from the start avoids frustration.
The Detail
Mil-spec buffer tube:
- Outer diameter: 1.148 inches
- Thread: 1.185 x 16 TPI (standard receiver extension threads)
- Finish: type III hard-coat anodized (matte, slightly rough texture)
- Material: 7075-T6 aluminum (stronger alloy)
- The industry standard — 95 percent of aftermarket stocks fit mil-spec
- Used by: BCM, Aero Precision, Daniel Defense, Geissele, all military rifles
Commercial buffer tube:
- Outer diameter: 1.168 inches (0.020 inches larger than mil-spec)
- Thread: same as mil-spec
- Finish: extruded and polished (smoother, shinier)
- Material: often 6061-T6 aluminum (slightly softer alloy, less expensive)
- Less common, primarily found on budget rifles
- Used by: some DPMS, older Bushmaster, budget brands
How to tell which you have:
- Measure the outer diameter: 1.148 = mil-spec, 1.168 = commercial
- Look at the finish: rough/matte = mil-spec, smooth/shiny = commercial
- Check the rear edge: mil-spec tubes are cut straight, commercial tubes are angled (sloped)
- The angled vs straight back edge is the easiest visual identifier
Compatibility:
- Mil-spec stocks on mil-spec tubes: perfect fit
- Mil-spec stocks on commercial tubes: will fit but may wobble
- Commercial stocks on mil-spec tubes: may be too tight or not fit
- Some stocks (Magpul MOE) come in both mil-spec and commercial versions
- High-end stocks (B5 SOPMOD, BCM Gunfighter) are mil-spec only
Buffer tube components:
- Buffer tube (receiver extension): the tube itself
- Castle nut: threaded nut that secures the tube to the receiver
- End plate: sits between the castle nut and the receiver, provides sling mount point
- Buffer: weighted cylinder inside the tube that absorbs recoil
- Buffer spring: compressed behind the buffer, returns the bolt carrier forward
- Stake the castle nut: after torquing, use a punch to deform the castle nut into the end plate groove to prevent loosening
Build Impact
For any new build, buy a mil-spec buffer tube. The Aero Precision mil-spec buffer kit ($30 to $40) includes the tube, castle nut, end plate, buffer, and spring — everything you need. Torque the castle nut to 40 foot-pounds and stake it properly. If you have an existing rifle with a commercial tube and want to run a mil-spec-only stock, swap the buffer tube rather than forcing the wrong stock onto it.