The ArmaLite Rifle 15, or AR-15, is America’s most popular semi-automatic centerfire rifle pattern. But can you confidently say that you understand how this piece of Americana — the “Black Rifle” — works?
The AR-15 has a unique operating system. If you don’t understand how your firearms work, you won’t be as prepared to use, maintain, and repair them. It’s also worth understanding how firearms work so that you can effectively argue against ill-conceived legislation that threatens your 2A rights.
How the AR-15 Works
Introduced commercially in 1964, the AR-15 is a semi-automatic, gas-operated rifle fed from a detachable box magazine. One unique feature of the AR-15, especially at the time of its introduction, was its straight-line construction.
As a direct result, the 5.56mm M16 — and its 7.62mm predecessor — was more controllable than rifles with more traditional stocks in fully automatic fire. There is less leverage, so it directs the recoil in a straight line to your shoulder.
While the AR-15 design is typically called “direct gas impingement,” the reality is somewhat more complicated. However, what Eugene Stoner’s design shares in common with direct-impingement systems is the elimination of additional moving parts. There is no operating rod or piston, and no off-axis parts impacting the bolt.
Check the correlation of AR-15 with M16 on this in-depth comparison guide.
Cycle of Operation
The cycle of operation in a repeating firearm comprises eight functions. These are described in relation to the AR-15 firing sequence:
When you squeeze the trigger, the trigger rotates around a pivot pin in the lower receiver. If the hammer is cocked, squeezing the trigger lowers its nose (i.e., the sear), disengaging it from the hammer. Free from the sear, the rotary hammer is driven forward by its own spring and delivers a blow to the firing pin.
The firing pin strikes the primer in the cartridge through the face of the bolt, crushing the priming material against the self-contained anvil. The primer contains an impact-sensitive primary explosive, so this action detonates it, which projects burning particles through the flash hole in the primer pocket. This ignites the main propellant charge.
Smokeless powder, being a low explosive, burns, generating high-temperature, high-pressure expanding gases that propel the bullet through the barrel. These gases also apply equal pressure in all other directions, expanding the case against the inside of the chamber and obturating (i.e., sealing) the breech.
See Related Article: Proper Way to Shoot an AR-15
When the bullet passes the gas port in the barrel, the powder gases enter the gas block, where they feed through the gas tube. The gas tube directs these gases into the bolt carrier group through the key, where they enter the gas cylinder formed between the bolt and bolt carrier. These gases exert pressure against the inside of the carrier, forcing it rearward.
As the bolt carrier travels rearward, the cam track in the bolt carrier acts against the cam pin in the bolt, rotating the bolt clockwise (when viewed from the rear) and unlocking it from the barrel extension.
The extractor is a spring-loaded claw-like part that snaps over the rim of the cartridge case when the bolt seats the round in the chamber. Its purpose is to remove the spent cartridge from the chamber. This part grips the case firmly, holding it against the face of the bolt. When the expanding gases force the bolt carrier rearward, it withdraws the bolt from the barrel extension, and extraction begins.
The AR-15 bolt uses a plunger-type ejector. This is a spring-loaded pin that the cartridge case head forces flush with the breechface as the cartridge is seated in the chamber. When the cartridge case is extracted, the plunger moves forward, canting the cartridge, which pivots around the extractor. Once the casing is free from the chamber, the plunger expels the case from the weapon through the ejection port.
On the rearward stroke, the bolt carrier group forces the hammer down, rotating it into the cocked position. The disconnector catches the hook of the hammer, retaining it until you release the trigger. When you release your finger from the trigger, the disconnector disengages the hammer, which is then caught by the nose of the trigger (i.e., the sear). The disconnector ensures the hammer does not follow the bolt carrier group during its cycle.
The rearward travel of the bolt carrier group compresses the buffer spring inside the receiver extension. When this spring expands, it forces the bolt carrier group forward. On this forward stroke, the bolt strips the top-most round from the magazine and feeds it into the firing chamber.
As the bolt enters the barrel extension, it fully seats the cartridge in the chamber. This is called chambering and occurs when the cartridge reaches the headspacing point — the point in the chamber that stops the forward movement of the cartridge. In 5.56mm rifles, this corresponds to the midpoint of the cartridge case shoulder.
The AR-15 is a locked-breech firearm, so the bolt locks behind the barrel to contain the explosion of the fired cartridge. This maintains pressure in the bore to drive the bullet and keeps the breech closed until the chamber pressure drops to a safe level.
The rifle locks by using a rotating bolt with seven radial lugs — the eighth position of the locking lug is occupied by the extractor.
When the bolt carrier continues forward, the cam track in the bolt carrier acts on the surfaces of the cam pin in the bolt. The cam pin rotates the bolt 22.5° counterclockwise, causing the bolt locking lugs to engage corresponding recesses in the barrel extension.
Piston-Driven AR-15 Rifles
Some argue that the AR-15 design is inherently piston-driven because the Stoner system uses the bolt carrier group as a “piston.” However, regardless of this interpretation, “piston” typically denotes a separate moving part that either impacts the bolt carrier (e.g., short-stroke pistons) or is attached to it (e.g., long-stroke pistons).
Another well-known ArmaLite design is the AR-18/180, which uses a short-stroke piston with a multi-piece operating rod to perform this action. While this weapon was never as successful as the AR-15, it did influence modern arms manufacturers. One result is the prevalence of AR-15-pattern rifles using a short-stroke piston system.
In these rifles, the gas key is a solid part that an operating rod strikes to facilitate unlocking. The common argument in favor of this system is that pistons are cleaner and cause less heat accumulation in the action.
Understanding how the AR-15 rifle pattern operates can help you effectively maintain or repair your weapon, ensuring that it affords you a lifetime of service. If you wish to pursue a gunsmithing career or become a hobbyist, an understanding of the mechanics can be crucial to your success.
However, a thorough working knowledge of how the function of your firearm is also important for another reason. It lets you confidently avoid and counter misinformation special-interest groups or the uninformed may disseminate.
The AR-15 rifle was an innovative product when ArmaLite introduced it almost 60 years ago. Its popularity has stood the test of time. To honor its legacy, you should understand how this classic weapon operates.
With the fast innovation of firearms nowadays, AR-15 pistols have been made to accomodate the state laws. To learn more about AR pistols, check out this definitive guide.
Learn more about the AR-15's common problems that every enthusiast encounters.
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