What the Spec: AR Specifications and Why They Matter
Introduction
You’ve seen it stamped on boxes, featured on product listings, and dropped in conversations like gospel: Mil-Spec. But what does it really mean? In the AR-15 world, the term gets thrown around so loosely it’s lost most of its weight.
Here’s the truth: Mil-Spec isn’t about branding. It’s about engineering. It’s a defined set of standards—materials, dimensions, finishes, testing—that govern how a rifle is made. These specs were developed for the U.S. military to ensure that any M16 part, from any vendor, would fit and function with any other. For builders and shooters who value reliability and interoperability, knowing what “the spec” actually is helps cut through the noise.
The Gold Standard: The Colt TDP
In the 1960s, the U.S. government adopted the M16 rifle. Along with it, they required a Technical Data Package (TDP)—a detailed blueprint that includes everything from heat treat specs to dimensional tolerances. Colt was the original TDP holder and primary contractor for decades.
This TDP is what defines “Mil-Spec” for the M16 and later, the M4. Unless a part was made according to that TDP—or a vendor had a government variance—you’re looking at a commercial-spec approximation.
If you are interested in seeing the TDP, check out our Spec to Inspect TDP collection:
What’s in the Spec? Key Categories
1. Material Specifications
Mil-Spec defines not just the shape of parts, but what they’re made from:
- Bolt Carrier Group: Carpenter 158 steel for the bolt, 8620 steel for the carrier.
- Receivers and Buffer Tubes: 7075-T6 aluminum, forged (not billet).
- Barrels: Chrome-moly vanadium (CMV) steel, not 4140 or stainless.
These materials were chosen for strength and durability. Commercial alternatives may look similar but often sacrifice fatigue life or structural integrity.
2. Surface Treatments
Finishes aren’t just about aesthetics:
- Phosphate (Parkerized) coatings are standard on carriers, barrels, and most small parts, offering corrosion resistance and oil retention.
- Chrome lining in the bolt carrier, chamber, and bore increases longevity and makes cleaning easier.
- Anodizing Type III hardcoat is standard on aluminum components.
Newer coatings like nitride or nickel boron might offer benefits for specific components, but they’re not part of the original spec…and they can introduce tradeoffs. The impact of alternate finishes/coatings can range from tempering of the metal to throwing off critical dimensions.
3. Dimensional Tolerance
One of the cornerstones of Mil-Spec is tight dimensional control. Interfacing components rely on compatible tolerances to ensure fitment and function. For example:
- Takedown/Pivot Pins and Lower Receiver: the pins should be 0.2480-0.2485″ and the corresponding receiver holes should be 0.250-0.252″.
- Buffer Tube Diameter and Buttstock: the carbine receiver extension tube diameter must be 1.143-1.149″ and the open end of the buttstock must have an inner diameter of 1.155-1.160″
- Gas Block Journal and Gas Block: the gas block journal on the M4 barrel must be 0.7488-0.7495″ and the front sight block bore must have an inner diameter of 0.7500-0.7507″
You can see that the Mil-Spec dimensional tolerances are tight and coordinated between interfacing components. Parts that are “close enough” can cause stacking tolerances, poor fit, or outright failures.
4. Manufacturing & Processing
Mil-Spec doesn’t just tell you what to make—it tells you how to make it.
- Thermal Processing of components includes hardening, tempering, and/or cryogenically treating the parts to ensure hardness, toughness, wear resistance, stress relief, and longevity.
- Shot Peening increases fatigue resistance in bolts.
- Gas keys must be sealed, torqued, and properly staked to spec.
All of these processes apply to every Mil-Spec component.
5. Testing and Quality Assurance
The Mil-Spec doesn’t let defective components onto the battlefield easily. The Mil-Spec includes quality testing and inspection of critical components.
- HPT (High Pressure Testing) of a bolt or barrel amplifies defects in the crystalline structure of the metal and ensures that the part can survive over-pressurization.
- MPI (Magnetic Particle Inspection) finds microscopic flaws invisible to the naked eye.
Unless otherwise specified, these quality processes are performed individually on every part. When you read “Mil-Spec,” ask: Was it tested like the military requires?
Mil-Spec vs. Commercial Spec: Why It Matters
Here’s a common mistake: assuming commercial-spec parts are “close enough.”
In practice:
- A commercial buffer tube won’t fit a true Mil-Spec stock.
- An untested bolt might survive plinking but shear under hard use.
- A hand-lapped stainless barrel might outshoot a chrome-lined one on paper, but fail to run clean in a field environment.
Mil-Spec parts are designed to be abused, swapped, and trusted—not just admired on a bench rest.
Be Wary of "Meets or Exceeds"
When a manufacturer says “meets or exceeds Mil-Spec,” ask:
- Was it made from the correct material? Some materials are actually better. For example: 4140 steel is better for the gas key than the Mil-Spec 4130 steel. However, if you are not a metallurgist, you might not be able to discern the differences to make an appropriate judgement call.
- Does the surface treatment meet or exceed the Mil-Spec? Consider adhesion, impact resistance, fracture toughness, fatigue resistance, lubricity, hardness, scratch resistance, and corrosion resistance.
- Was it built the right way? Some manufacturers can make a component sparkle, but don’t go through the motions to build correctly. For example: a fancy coating on a BCG is less important than properly sealing and staking the gas key.
- Was it tested the same way? Batch testing is not the same as individual testing. For example: batch HPT is not acceptable—microscopic flaws in the crystalline structure do not occur at the batch level.
- Was it held to the same tolerance and QA requirements? There are very few scenarios when a dimensionally out-of-spec component any benefit over the Mil-Spec. Gauging of critical dimensions is absolutely essential, but often not performed in the consumer market.
Unless the part was built to the TDP or manufactured on a known military contract, the claim is mostly marketing.
How to Spot a True-Spec Part
- Look for specifics, not vague claims. Example: “C158 bolt, HPT/MPI, shot peened, phosphate finish”
- Ask about testing: Was it batch tested or individually proofed?
- Know your manufacturers: buy from manufacturers who understand and commit to the Mil-Spec. You have two safe options:
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- Buy from a manufacturer that discloses their detailed specifications, which can be compared against the TDP.
- Buy Mil-Spec components from defense contractors (Colt, FN, LMT, Daniel Defense). Note that consumer marketed “Mil-Spec” products from these manufacturers do not necessarily meet the same quality standards as those sold to the U.S. Government (you may be getting third round rejects that can’t be sold to the Government).
Additional Resources: Learn More About AR-15 Specifications
- Spec to Inspect: Deep Dive Series
Our multi-part technical series explores how to inspect and evaluate AR components according to military specifications. Perfect for builders who want to verify parts using real measurements and metallurgy. - What Is the TDP? A Closer Look at the Military’s Blueprint for the M16/M4
The Technical Data Package (TDP) defines what “Mil-Spec” really means—including materials, dimensions, finishes, and testing protocols. This article breaks down how it governs interoperability and reliability. - AR-15 Compatibility Guide
Learn which components are interchangeable across AR platforms (pistol caliber, small frame, large frame) and how spec differences affect fit, function, and reliability. - AR-15 Barrel Selection Guide
Explore how barrel specs—length, gas system, materials, finishes—affect your rifle’s performance and compatibility.
Final Thoughts: Why It Pays to Know the Spec
If you’re building a duty-grade AR, clone-correct rifle, or simply want gear you can trust when it counts, Mil-Spec isn’t optional—it’s the foundation. Understanding what makes a part spec-compliant helps you separate legit components from lookalikes that cut corners.
The spec exists for a reason. Honor it. Build to it. Demand it.
BONUS: 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 (Manufacturing & Processing)
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 (to them).
The threading process has more of an effect than it may seem. And with that, let’s look at the differences in dimension…
Dimensional Tolerances
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" |
In general, Mil-Spec and commercial spec components are NOT cross-compatible (with limited exceptions).
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. Enter the first significant incompatibility:
- 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.
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. Enter the second major incompatibility:
- 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 these primary Mil-Spec and commercial spec components is summarized in the table, below:
The mating of the commercial spec receiver extension threads and Mil-Spec castle nut threads will not be ideal. The two will mate loosely. We are not aware of any commercial spec castle nuts. So, be sure not to overtighten the castle nut if using a commercial specification tube; doing so may strip the threads of the receiver extension.
You shouldn’t have any issues with the receiver extension end plate. We are not aware of any commercial spec end plates.
Verdict: Don't Use Commercial Spec
Commercial spec lowers are rare today—and for good reason. 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.
Frequently Asked Questions
Why does Mil-Spec matter when building an AR-15?
Why is Mil-Spec important for AR-15 reliability?
How do Mil-Spec standards affect AR-15 parts compatibility?
Mil-Spec ensures that parts like uppers, lowers, bolt carriers, buffer tubes, and barrels from different manufacturers will fit and function together. If you’re mixing components, staying Mil-Spec is the safest path to guaranteed compatibility.
What’s the difference between Mil-Spec and commercial spec AR-15 parts?
Mil-Spec parts are built to the U.S. military’s Technical Data Package (TDP), which includes strict standards for dimensions, materials, and testing. Commercial spec parts may deviate from these standards—especially in areas like buffer tube dimensions—and are not guaranteed to be interchangeable with Mil-Spec components.
Should I build my AR-15 with Mil-Spec or commercial spec components?
If you want maximum reliability, compatibility with a wide range of parts, and future upgrade potential, Mil-Spec is the safer bet. Commercial parts may offer cheaper options or novel designs, but you sacrifice interchangeability and often long-term durability.
What are the Mil-Spec dimensions and tolerances for AR-15 components?
What materials are used in true Mil-Spec AR-15 components?
Mil-Spec upper and lower receivers are made from 7075-T6 aluminum forgings, not the more common 6061-T6 used in cheaper builds. Bolt carriers use Carpenter 158 bolts and barrels are typically CMV steel, not 4140.
Every component in the weapon system has acceptable materials specified in the TDP.
What’s the difference between manganese phosphate and nitride finishes?
Manganese phosphate is the Mil-Spec standard, offering good corrosion resistance when properly oiled and creating a rough surface that holds lubricant. Nitride (melonite/QPQ) is a modern alternative that offers higher surface hardness and corrosion resistance but is not part of the original military specification.
Every component in the weapon system has acceptable finishes specified in the TDP. It is worth noting that the TDP explicitly prohibits nitride for the bolt (see our Don’t Buy a Nitride Bolt article for more information).
What are common Mil-Spec heat treatments and surface processes?
The thermal processing (heat treatment, tempering, cryogenic treatment), surface treatment, testing, and inspection of components are specified in the TDP. AR-15 bolts are shot-peened to improve fatigue resistance, and are high-pressure tested (HPT) and magnetic particle inspected (MPI) to detect flaws. Barrels are often stress relieved and chrome-lined to ensure durability and maintain accuracy.
What do HPT and MPI mean for AR-15 bolts and barrels?
High Pressure Testing (HPT) involves firing a proof round to ensure the bolt or barrel can handle overpressure without failure. This will stress any imperfections and occlusions, allowing the defect to be visualized.
Magnetic Particle Inspection (MPI) uses magnetization and iron particles to find microscopic cracks.
Both tests are required for Mil-Spec bolts.
Are AR-15 takedown and pivot pins different between Mil-Spec and commercial lowers?
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