Used frequently among owners of the AR-15 and its variants, “mil-spec” is a term that has achieved practically god-like status. If something is mil-spec, many assume, it must be of the highest quality.

But what is mil-spec, and why does it matter? It’s used so often that the truth has been buried in myth, but understanding mil-spec, both what it is and what it is not, will help you achieve greater enjoyment from your rifle.

Military specifications, commonly called “mil-spec,” as well as “defense standard,” and “military standard,” is a system used to achieve the standardization of tools, materials, and weapons used by the United States Armed Forces. United States Military Standards are set by the Department of Defense, and they are crucial to ensuring materiel achieves a certain level of requirements.

One of the most important elements of military specifications is the concept of “interoperability,” which is the ability of certain products or systems to function together and work with other products. For example, the Department of Defense may designate the specifications of a tiny screw used in Armed Forces weaponry. The hypothetical screw can fit into multiple weapons, and all (or most) weapons are engineered to use this screw. Having one screw that is used in multiple weapons increases the efficiency and cost-effectiveness of manufacturing military equipment.

Mil-spec, therefore, is any tool, weapon, vehicle, computer system, or component that has been designated and approved by the Department of Defense. A spoon could, in theory, be mil-spec if the Department of Defense says so. The spoon may not be better than any other spoon, but because it’s the one chosen by the DoD, it is officially mil-spec.

Mil-spec equipment began with a need to have proper performance and repairability among all military equipment. While standardizing military equipment dates back centuries (Roman shields were standardized), it’s believed that the American and French military forces were the first modern powers to make standardization a significant priority. Interchangeability and standardization were, for example, used extensively in the American Civil War. The Springfield rifle loaded a ball of .58 inches (.58 caliber); this standardization ensured that bullets produced for the military would always fit properly, and that all sections of the Union military, be it the Army of the Potomac or the Army of the Tennessee, could use the ammunition.

In World War I, which saw rapid technological advancement, the need for standardization became clear, and by the Second World War, there were not just military standardizations within an individual nation, but standardization between allies.

The U.S.-British alliance is a perfect example of international military standardization: at the beginning, American components did not fit British materiel and vice-versa. Due to dimensional differences, you could not use a bolt from a ruined American rifle to repair a British weapon. You could also not use ammunition interchangeably. So if an American company was running low on ammo but linked up with British allies, the Brits would not be able to resupply their American friends. You don’t need to graduate from West Point to understand why this was a significant issue for our fighting forces, and why standardization, both nationally and internationally, became a serious priority for America’s top brass.

Since the global conflicts of the early 20th century, military standardization has been crucial, and the M16 rifle became the perfect example. This rifle, which saw its first significant use in the Vietnam War, is highly standardized by the Department of Defense. It has specific and precise measurements for weight, height, length, caliber, barrel width, spring size, sights, and more. Not just the rifle, but all of its accessories used by U.S. fighting forces, are under extreme standards of military specification. This rifle is not just used by the U.S., but dozens of American allies, ranging from Belize to the United Kingdom.

But what about the civilian equivalent of the M16: the AR-15 and all of its variants?

While the AR-15 is not mil-spec, (and essentially never can be), with modifications this firearm can come close.

Just because your AR-15 isn't Mil-Spec by the strict definition of the term does not mean components of your rifle can't be Mil-Spec.

First of all, we need to establish one important principle: the AR-15, or any of its variants and copies, cannot be completely mil-spec. The specifications for the M16 call for the weapon to be fully automatic, with switches for bursts of fire. This would be illegal in a civilian firearm.

In order for an AR-15 to be 100% mil-spec, it needs to be fully automatic. For civilians, fully-automatic weapons are illegal (with some exceptions). So there you have it: your AR-15 simply can’t be mil-spec, at least not by a strict definition of the term.  

But that doesn’t mean certain components of your rifle can’t be mil-spec.

You can, for example, add grips, stocks, and other components that would make your rifle closer to mil-spec standards. You can, essentially, equip your rifle entirely with parts and components that meet military standards, excluding the components that make the rifle fully automatic.

When comparing mil-spec to commercial components, standardization again becomes crucial. With military-spec equipment, you can be assured that you are getting interchangeable parts and accessories that make installation more convenient and the use of the rifle more effective. Mil-spec components can be more expensive, but you will likely find more options that fit with the specific item. Basically, if you stick to mil-spec for all of your aftermarket parts, you will have greater compatibility for your entire weapon. With commercial parts, there can be vast differences in the size or fit, which means matching and interchanging the components can be complicated.

As we’ve described, mil-spec is an important consideration for owners of AR-15 components because it helps standardize the accessories and components you use, creating better convenience. There is also the theory that, because mil-spec also includes the materials used to manufacture an item, they can be more reliable. (The U.S. military rarely uses low-quality materials.)

So while you may not be able to achieve 100% mil-spec on your AR-15, as you come closer, you will increase the convenience and effectiveness of your rifle.