What the Spec: AR Specifications and Why They Matter
Short Story: Mil-Spec ensures consistency, compatibility, durability, and performance. Make sure you use components that meet (dimension) or exceed (material, finish, treatment, performance) this standard. Don’t use commercial spec components if you don’t have to.
The Military Specification (Mil-Spec)
Interestingly, the most important specification to bear in mind with the civilian AR market is the Military Specification, or Mil-Spec.
The civilian AR is derived from its military predecessor, the M16. The M16 (as with any military product) has a ton of specifications. Every dimension, angle, material, finish, etc. has clear specifications outlined in the Colt Technical Data Package (TDP).
The Mil-Spec for the M16 is born out of the U.S. government’s expectations of 1) reliability and durability, and 2) interchangeability and compatibility (regardless of lot, manufacturer, etc.) for their weapon systems. As such, there is a lot of value to Mil-Spec.
There are five primary attributes of the military specification for the M16 and M4:
Material
This is the most fundamental specification of a component and is a determinant of it’s basic physical properties. Each component has an associated material specification. Examples of Mil-Spec materials include 7075 T6 aluminum or Carpenter 158 steel.
Finish
An extension of material, the finish refers to the chemical or physical coating on the exterior surfaces of a component and determines the resistance of the component to external forces and reactions, like friction, wear, and oxidation. Relevant examples of finish include manganese phosphate, nitride, oxide, chrome, Cerakote, DLC.
Processing
An extension of material, (manufacturing) process includes any treatment the component material. This may include shot peening, staking, cryogenics, heat treatment, threading, etc.
Dimension
Dimensions (and tolerances) refer to the size of a component, and are essential to the standardization and compatibility of components between firearms. For example, the lower receiver pivot and takedown holes on the military version of the AR-15 must be 0.250″ +/- 0.002″ and the pivot and takedown pins must be 0.250″ +/- 0.002″. If either component is out of specification, they will not fit together.
Testing/Inspection
Performance inspection and testing ensures the component is sound and meets other specifications. Examples include magnetic particle inspection (MPI), test cycling/firing, etc.
Mil-Spec: The Bare Minimum
Generally, Mil-Spec should be your minimum acceptable standard. If it doesn’t meet or exceed Mil-Spec, don’t use it.
Some attributes must meet Mil-Spec, exactly (no acceptable deviation). Examples include dimensions of interfacing components: if your pivot pin has a diameter out of tolerance, it will not play nice with your receiver (and vice versa).
Some attributes can exceed Mil-Spec. This generally applies to material and finish, but can also apply to treatment and performance testing. For example, if the Mil-Spec pivot/takedown pins are steel, changing to titanium pins may be considered an improvement. Similarly, if the Mil-Spec for a component’s finish is manganese phosphate, then nitride, hard chrome, or Diamond-Like Carbon (DLC) may offer a substantial improvement (as long as the finish does not push critical dimensions out of tolerance).
We strongly recommend that you focus on achieving Mil-Spec with your builds. Be cautious about the civilian industry’s proclivity for “meaningless” buzzwords (like “Mil-Spec”). Civilian component and firearm manufacturers can claim that their products meet the Mil-Spec, even when they don’t (they aren’t selling to the Government, so nobody is checking them). Our best advice: buy from companies that can provide detailed specifications for their products, or those who manufacture the same part for the Government.
The remainder of this article dives into a specific component: the receiver extension.
Commercial Spec: The “Other” Specification
For some reason a long time ago, as the civilian variant of the M16 hit the consumer market, component manufacturers deviated from the military specification for the receiver extension (a.k.a. buffer tube). It doesn’t matter why (*cough* penny-pinching), but it had a profound impact on the aftermarket industry.
The differences can be detected in three of the five attribute categories:
Material
Short Story: Commercial Spec uses inferior materials.
The primary deviation from the Mil-Spec was the material used.
The Mil-Spec receiver extension is forged from 7075 aluminum alloy.
The commercial specification receiver extension is machined from 6061 aluminum alloy.
The impact: 6061 is great for the manufacturer, but inferior to 7075 in all of the ways that matter most to you, the consumer:
| Attribute | Mil-Spec (7075 T6) | Commercial (6061) |
|---|---|---|
| Machinability | Fair | Good |
| Cost | Higher | Lower |
| Hardness (Brinell) | Harder (150 HB) | Softer (95 HB) |
| Ultimate Strength | Stronger (83,000 psi) | Weaker (45,000 psi) |
And, even if a commercial spec receiver extension is manufactured using 7075 T6 alloy, there are other problems…
Threading (Processing)
Short Story: Commercial spec uses an inferior threading process that results in shorter and weaker threads.
The next major deviation was the threading process.
As part of the Mil-Spec manufacturing process, the threads are rolled into the receiver-end of the tube, meaning the threads are extruded/displaced into form (as opposed to cutting, where material is removed to form the threads). Not that it is particularly impactful in this context, but rolling has its advantages over cutting; generally it produces stronger thread (typically about 30% increase in ultimate strength due to “work hardening”), taller threads (as they are extruded outward), and more consistent precision.
The early manufacturers of components of the civilian variant figured out that they could save money on machining cost by cutting the threads into the receiver extension, instead of rolling them. While this produced weaker and shorter threads and generated more waste from the cuttings, the cost savings to the manufacturers outweighed the drawbacks.
The threading process has more of an effect than it may seem. And with that, let’s look at the differences in dimension…
Dimension
Short Story: The Commercial spec tube has an increased thickness and outer diameter, but narrower thread diameter.
The Mil-Spec standard diameter and thickness for the receiver extension tubing are 1.1475″ +/- 0.0025″ and 0.073″, respectively. Because the threads are extruded during the rolling process, the diameter at the threads is actually greater than the starting diameter of the tube. The outer diameter of the threads is 1.185″.
The manufacturers defining the commercial spec realized that they needed to compensate for the weaker material and threads by increasing the thickness of the tubing. They had two options: decrease the inner diameter, or increase the outer diameter. They went with the latter option. The commercial spec tubing diameter and thickness are 1.168″ +/- 0.005″ (about 0.02″ larger than Mil-Spec) and 0.085″, respectively. Because the threads are cut into the tubing, the outer diameter of the threads is roughly equal to the outer diameter of the tubing. At the threads, the commercial spec receiver extension is 1.170″ (about 0.015″ smaller than Mil-Spec).
The important dimensional differences are summarized in the table, below:
| Specification | Tubing Diameter | Thread Diameter |
|---|---|---|
| Military | 1.1475" +/- 0.0025" | 1.185" |
| Commercial | 1.168" +/- 0.005" | 1.170" |
There are other dimensional differences, but these are the big ones that have the most impact on the system and component selection.
The Big Picture: Mil-Spec vs. Commercial Spec
Because of the dimensional deviation of the commercial specification receiver extension at the threads, the lower receiver needed to be adjusted to allow the use of the smaller thread diameter of a commercial spec receiver extension. So, manufacturers adjusted the specification of the lower receiver.
Because the tubing diameter was larger than the Mil-Spec version, the buttstock would not fit over the receiver extension. So, they created a dedicated line of buttstocks that would fit onto the larger receiver extension tubing.
Generally, you shouldn’t have any issues with the receiver extension castle nut or end plate; we are not aware of any commercial spec castle nuts or end plates. That said, there may be exceptions floating around out there.
Mil-Spec and commercial spec components are generally NOT cross-compatible.
A Mil-Spec receiver extension will not fit into a commercial spec lower receiver, and a commercial spec receiver extension will generally be loose in a Mil-Spec lower receiver.
A Mil-Spec buttstock will not fit onto a commercial spec receiver extension, and a commercial spec buttstock will be loose and wobbly on a Mil-Spec receiver extension.
The compatibility between Mil-Spec and commercial spec components is summarized in the table, below:
| Receiver Extension Specification | Mil-Spec Lower Receiver | Comm. Spec Lower Receiver | Mil-Spec Buttstock | Comm. Spec Buttstock |
|---|---|---|---|---|
| Military | Yes | No | Yes | Loose |
| Commercial | Loose | Yes | No | Yes |
Verdict: Don’t Use Commercial Spec
Short Story: Don’t buy commercial spec if you aren’t already stuck with it.
We’re not even sure where you can get a commercial spec lower receiver nowadays, but if you are ever faced with the option, don’t buy one.
All else being equal, the most significant issue you will face with commercial spec is the limited availability of commercial spec options for the affected components.