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Understanding Mil-Spec AR-15s: What You Should Know

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If you have spent any time looking into AR-15s or AR-15 parts and accessories, there is a good chance you have seen components referred to as “Mil-Spec.”

What does Mil-Spec mean, and why is it important? The short answer is Mil-Spec stands for “Military Specification.” The U.S. military sets these standards to ensure that every part has the same quality and size, allowing for easy interchangeability of parts.

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What is a Mil-Spec AR-15?

First, let’s dive deeper into what Mil-Spec means. Mil-Spec is a document that sets forth requirements that all goods sold to the U.S. military must adhere to. Every good sold to the military must meet these requirements, from tires and clothing to firearms and ammunition.

This standardized specification makes sure that every member is armed with gear that is of the same quality and can interchange parts. Not only is this great for ensuring a baseline for the level of quality, but it also simplifies logistics.

While a civilian AR-15 may be functionally different from a military service rifle, many of the parts share the same dimensions.

In the civilian world, the term Mil-Spec has seemingly become a marketing tool. Mil-Spec should be used to set the standard for the baseline quality level of firearms and parts. While these standards will result in a reliable firearm, they are still far from the best that can be achieved.

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The most significant advantage of these specifications is the peace of mind regarding AR-15 upgrades or replacement parts. If you have a Mil-Spec receiver, you know that any parts, whether a trigger or simply a detent spring for a safety selector, will fit and adequately work if they too are Mil-Spec.

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Mil-Spec Quality

While military standards are not the gold standard in quality, they are very well-tested. These standards are derived from stringent testing, ensuring durability and performance. Again, these standards are excellent, but may not be the best money can buy.

Let’s look at barrel finishes as a good example. Currently, the military standard for barrel finishes is chrome lining. Chrome lining is a finishing process completed by using an electrical current to bind chrome to the inside of the barrel.

Chrome lining provides excellent durability and corrosion resistance and has been used for more than 50 years in firearm barrels. That said, there are plenty of other barrel treatment options on the market today that are just as accurate and durable, some may argue that there are even better options out there. However, chrome lining has been time-tested and proven to be an excellent option for a highly accurate and long-lasting barrel.

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Other considerations included within these military standards lie in the QC and the individual parts’ inspection. An example of this is the military requires all bolts to be MPI tested.

MPI stands for magnetic particle inspection and is the process of detecting surface and subsurface flaws. This is achieved by putting a magnetic field into the part. The part can be magnified by passing an electrical current forming a magnetic field. If there is a flaw, the magnetic field will leak.

Mil-Spec AR-15 Accessories

Like we said earlier, one of the best parts about Mil-Spec is that you can easily change parts and accessories without worrying about them fitting.

So, will all accessories work with your AR-15? Short answer, not exactly. If the accessory, like a stock for example, is Mil-Spec and your buffer tube that the stock attaches to is also Mil-Spec, you’re good to go. However, if you have a commercial-sized buffer tube and a Mil-Spec stock, and vice versa, these two parts may not be compatible.

In this example, the commercial buffer tube is roughly 0.02″ wider than a Mil-Spec buffer tube, so the stock, depending on its tolerances, may not be able to fit.

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Be sure to double-check that all the AR-15 upgrades you may be adding are compatible with your rifle. It is best to save yourself from the inconvenience of returning or replacing parts if you can easily avoid it.

If you are unsure you can always reach out to our expert team via chat, email (, or phone 713-344-9600, and we would be happy to make sure you are getting the right parts.

Let’s Wrap it Up

We now know that if a firearm or firearm part is mil-spec, it meets the standard of quality set by the U.S. military. Meeting Mil-Spec can mean the firearm or part is far above standard or just skating the line. If you want the best of the best, it is wise to dig a bit deeper into what makes the best AR-15. Check out our article about What is the Best AR-15 for Your Needs.