has come a long way. When Parsons and Sikorsky decided to numerically control machines that made punches for helicopter rotor parts, they quickly realized that they didn’t know a lot about computers. MIT took the project in hand and made the first NC machines out of old Cincinnati mills. At the time, it literally was Numerical control; the circuitry took numeric instructions and made the servomotors move. Not until several years later did they actually integrate memory into the machines making them true Computer Numerical controls.

Without the CNC machine, a powerful lot of stuff the military needed could not be made, and so in concert with MIT, the Military purchased and retrofitted some machines and provided them to some of the shops they used, so they could get the stuff they needed.

The AR grew up in this time. NC and later CNC machining made the AR platform not only possible but fairly easy to build- it did for firearms what Henry Ford did to cars. It was no longer necesary to be a competent gunsmith or even machinist to make a good functioning firearm, pretty much any cletus could put a forging into a machine and a serviceable part would come out.

This was also sort of true of other firearms, like the AK, which, while it was not the child of CNC certainly benefitted by it and still does. The difference is the AK is designed to have wide open tolerances which allowed it to operate under many conditions, and withstand almost unbelievable abuse; the tradeoff is marginal accuracy, and pretty much always will be.

Unlike the AK, the AR was designed to be a precision firearm with precision fitted components, but with the ability for tolerance stack and human error removed.

Remember the post about Tolerance Stack? The brilliant thing about a CNC machine is that once a part has been verified, and tested exclusively, it can be duplicated with precise and repeatable accuracy almost infinitely. A properly cared for machine tool with appropriate tooling and careful operation has an indeterminate life; there really isn’t any way of telling how long one will last. Many of my customers are still using equipment made by Pratt Whitney and Giddings and Lewis that are now fifty or more years old. The leadscrews have automatic lubrication, they ways and gibs adjust readily and easily, and as long as they are not abused they continue to make good parts.

Stoner knew this, and he designed the components of his firearms to be easy to make with high precision. Let’s look at one of the most critical parts- though a lot of AR owners don’t even know it’s there.


You can click on the image to embiggenate.

This is the barrel adapter. Most people never see it as a separate part of their barrel, but if you look up into your receiver you should be able to see the threads that join it to the barrel inside the adapter.

This little bastard is brilliant. I mean, bloody brilliant.

In a bolt action rifle, or in other types of autos, the receiver is a solid machined piece that has the barrel screwed or pinned or sweated into the receiver face. This in itself requires a good deal of hand fitting, but the additional issue of fitting the bolt in place, making sure the bolt hits the recever lugs in the right place, and doesn’t allow excess headspace, is a good damned deal of work for even a talented gunsmith.

This little chunk of steel makes all that go away. You see, in a CNC machine, when you cut a thread, the thread always begins and ends in the same place, every single time, always. Which means that when you CNC machine a barrel, it’s threads will always start at, say, nine o’clock, and end at eleven o’clock. The threads in the barrel adapter are similarly oriented.

When the barrel adapter is made, it is machined in two operations, on a CNC machine. First, the face is squared, giving you surface A. Then the material is bored out of the center, the bottom of the hole faced, (Surface C)and the threads are cut. Then the outside of the part is machined, giving you diameter D and surface B. The part is then cut off and the secondary operations are performed. This means several things.
1: The surfaces A, B, and C are always parallel to one another, to the limit of the machine’s accuracy.
2: The surfaces A, B, and C are perpendicular to the diameter D.
3: Diameter D is concentric with the root diameter of the thread T
4: The thickness between surface A and B and the distance between surface A and surface C is predictable, repeatable, and consistent from one part to another.

Far better than can ever be done by hand in multiple operations, and far better than the average gunsmith wants to do. When JMB, God rest his soul, designed the M2 Garand (thanks Tam), he made it so the hand fitting was easy and simple. When Stoner made the M16, he made it so no hand fitting was required, period.

The upshot of this is subtle but vital. Barrels are made in a bunch of different ways, but at the end, the last thing that happens is that they get threaded and chambered. Because of CNC manufacture, threading the barrel into the barrel adapter means that the headspace is part of the barrel and adapter, and the headspace does not depend on the receiver but moves entire as a unit from receiver to receiver. Thus, a barrel/bolt that is a good accurate combination at a specific headspace can be moved from one firearm to another and the accuracy of the barrel moves with the barrel, assuming it is placed in a properly manufactured firearm. Here’s the subtle bit: No hand fitting is required to get the relationship between the barrel and the barrel adapter just right, because it’s done in CNC machines, and if done to spec it will always be just right. Sure, it’s possible to get a match grade barrel that’s headspaced as tight as a drum, but even then, you’ll see that they are sold as sets. You get the barrel, the adapter, and the bolt. Therein lies the accuracy of the firearm, and nothing else can effect it like those components can.