You've heard the phrase way too many times now: "Mil-spec." It's mostly a buzzword but it still plays a role in actual rifle and pistol specifications. Most notably, buffer tubes. They're officially called receiver extensions. But what's the difference between a commercial and mil-spec tube? Are they interchangeable? And how does that work with a commercial or mil-spec lower receiver?
Commercial vs. Mil-Spec Tube Diameters
The first problem with commercial and mil-spec tube compatibility is the physical size differences between the two:
- Mil-spec buffer tubes have an outer diameter of 1.148"
- Commercial tubes have a larger outer diameter of 1.168"
Just as important is the fact that all buttstocks and pistol braces are sized up for either commercial or mil-spec tubes, but not both. If you attempt to install a mil-spec buttstock on a commercial buffer tube, it won't fit. If you try the inverse, the buttstock or brace will be loose and wobbly on the tube, and it might not lock into position to set a good length of pull.
Commercial vs. Mil-Spec Threads
Someone out there is entertaining the idea of trying to fit a mil-spec tube onto a commercial receiver. But it can't be done, because the threads are also different on both tubes:
- Mil-spec tube threads are 1-3/16-16 UN rolled threads.
- Commercial tube threads are 1-3/16-16 UN cut threads.
Rolled threads are taller than cut threads. They sit above the outer diameter of the tube itself. Cut threads are recessed into the tube. So, even though commercial and mil-spec tubes' threads match, one set of threads is still physically larger than the other. If you tried to thread a forged tube into a commercial lower receiver, it would either cross-thread or simply not threat at all. If you threaded a commercial tube onto a forged receiver, it would be wobbly and the threads would likely "jump" or skip and potentially bind up when turned.
Which Tube is Better?
You know the two important pieces: Both tubes have different dimensions and technically different threads, but the differences don't end there. Aluminum AR-15 parts that are considered mil-spec must be forged from 7075-T6 aluminum. Commercial AR-15 parts are CNC-cut from 6061-T6 billet aluminum. Forged units will always be stronger and more well-formed than commercially CNC'd units, whether it's a receiver, tube, handguard, or other component.
Why Do Commercial Tubes Exist?
One reason: Cost.
Forging a tube with threads means investing in complex equipment that can melt metal, and exert high amounts of heat and pressure with high precision. But simply cutting a buffer tube and some threads out of a raw block of material with a small CNC machine is quick and easy. Some manufacturers use nothing more than a lathe, which costs a fraction of a smelter and forge. That's truly the only reason for why commercial buffer tubes exist. Of course, that's not to say they can't offer some advantages over mil-spec tubes now that they're on the market.
Not all receivers are mil-spec
Today, most lower receivers -- even those not forged but instead CNC'd from billet -- have mil-spec threads so they can accept mil-spec buffer tubes. But other lower receivers are on the market that have unique designs, custom fasteners and commercial specs. If you find that special billet receiver you just have to have because it has that rollmark, then a commercial tube may be required to complete the build.
Do Both Tubes Use The Same Parts?
Yes. No matter whether you're running a commercial or mil-spec buffer tube, the internal components -- buffer, recoil spring, bolt carrier, and buffer retainer -- remain the same. The differences in tube diameter are not great enough to warrant making custom springs and buffers. All lower parts kit parts fit in either tube just fine.
Summary: Which Tube is Best?
Unless you've found that special receiver that requires a commercial tube, stick with mil-spec buffer tubes. They provide too many advantages in the way of better fitment, higher strength, and more parts availability than commercial tubes and receiver combos.