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How it works: inside the AR-15

Since its fielding, the M16 earned a reputation of being very accurate and controllable during fire (reliability and quality control issues still wereto come...): in the image, a US 101st Airborne Division trooper responds to enemy fire in Quang Ngai in Viet Nam

All gun enthusiasts know how the AR-15 / M16-M4 and its “Stoner system” works, right? However, do they? If you wonder how the AR-15 works, you will find answers to your questions here, and even those who think they have it clear may find a couple of surprises before they are finished reading

Marco Dell'Acqua

When Eugene Stoner designed the AR-15, he was obsessed with the in-line concept: everything had to happen on the same axis, which went from muzzle to stock. It wasn’t just on a mere whim: the purpose of this arrangement was to minimize any shaking and any force acting on the weapon which didn’t go straight back against the shoulder of the shooter, to avoid disturbing the point of aim and allow for fast follow-up shots. 

All gun enthusiasts know how the AR-15 / M16-M4 and its “Stoner system” works, right? However, do they? If you wonder how the AR-15 works, you will find answers to your questions in this article
All gun enthusiasts know how the AR-15 / M16-M4 and its “Stoner system” works, right? However, do they? If you wonder how the AR-15 works, you will find answers to your questions in this article

Most semiautomatic weapons have a lot of moving parts, which run above, or under the barrel of the gun, which in turn is usually on a different axis than the stock. 

Each time a force acts on a line different from the axis that passes from the contact point on the shooter’s shoulder, it creates a rotating force that will move the muzzle away from its intended aiming point. In the AR-15 all forces are applied on the same axis and are directed either straight forward or straight back. 

How the AR-15 works


To make this possible, Stoner devised a system where a small part of the hot gases which propel the bullet is diverted from the barrel and brought directly to the Bolt Carrier Group (BCG) without intervening mechanisms. 

AR-15: the bolt carrier group (BCG)  strips a cartridge from the magazine
AR-15: the bolt carrier group (BCG) strips a cartridge from the magazine
AR-15: the cartridge is pushed in the breech by the BCG
AR-15: the cartridge is pushed in the breech by the BCG

The AR-15 fires from a locked bolt, meaning that when a cartridge is chambered, the bolt closes behind the case and rotates 15 degrees: the lugs on the bolt head engage the protrusions on the barrel extension, solidly locking the bolt in place, so it can withstand the high pressure produced by powder combustion. 

AR-15: The bolt rotates forced by the cam pin, locking the bolt in the barrel extension
AR-15: The bolt rotates forced by the cam pin, locking the bolt in the barrel extension
AR-15: The gas bleed from the barrel inside the BCG thru the gas tube. Here, the gas start to exert pressure on all of the surfaces in the expansion chamber
AR-15: The gas bleed from the barrel inside the BCG thru the gas tube. Here, the gas start to exert pressure on all of the surfaces in the expansion chamber

When the cartridge is fired, the bolt needs to rotate in the opposite direction and pulled back, so that the spent case can be extracted, ejected and a fresh round can be chambered. 

AR-15: The bolt rotates and unlocks, while the BCG continues in its rearward travel, extracting the spent case and cocking the hammer
AR-15: The bolt rotates and unlocks, while the BCG continues in its rearward travel, extracting the spent case and cocking the hammer
AR-15: The spent case is ejected; the main recoil spring pushes the BCG forward again chambering a fresh round and the cycle restarts
AR-15: The spent case is ejected; the main recoil spring pushes the BCG forward again chambering a fresh round and the cycle restarts

As soon as the bullet passes the gas port in the barrel, part of the hot gases start to flow into the gas tube towards the BCG. Here they enter the expansion chamber through the bolt carrier key and start to exert pressure on all of the surfaces in the expansion chamber, whose forward surface is the piston in the bolt tail, with its three C rings. 

How the direct gas impingement system of the AR-15 actually works: it is obvious that the gas expands in a chamber formed by the rear portion of the bolt and the internal cavity of the BCG
How the direct gas impingement system of the AR-15 actually works: it is obvious that the gas expands in a chamber formed by the rear portion of the bolt and the internal cavity of the BCG

These are not different from the gas seals on the pistons in the engine of your car. As in a car engine, the gases push on this piston, only in this case it’s the cylinder that moves from a stationary piston: the bolt carrier is therefore pushed back. 

Two things happen: first, the rearward travel of the bolt carrier makes the two holes drilled on its side pass by the sealing C rings placed on the tail of the bolt, venting the hot gases. 

A disassembled bolt and BCG of an AR-15: note the sealing C rings placed on the tail of the bolt and the chrome lined inner chamber of the BCG
A disassembled bolt and BCG of an AR-15: note the sealing C rings placed on the tail of the bolt and the chrome lined inner chamber of the BCG
Sectioned view of an AR-15, detailing the relationship of the BCG with the gas tube and bolt
Sectioned view of an AR-15, detailing the relationship of the BCG with the gas tube and bolt

Second, the bolt carrier continues to move backwards due to inertia, and the cam slot milled into its top side acts on the bolt lug, forcing the bolt to rotate into its unlocked position. By this time the bullet has left the barrel for good and pressure has dropped to safe levels. As the bolt carrier continues its rearward movement due to inertia, the bolt is pulled back from the breech, the spent case extracted and ejected. The recoil spring, which has been compressed by the BCG, pushes the bolt carrier forward again, stripping a fresh cartridge from the magazine, chambering it and, through interaction of the cam slot and bolt lug, the bolt is rotated into locked position again. 

Sectioned diagram of an M16A1 assault rifle. Note the perfectly coaxial placement of the action with the barrel and stock
Sectioned diagram of an M16A1 assault rifle. Note the perfectly coaxial placement of the action with the barrel and stock

This arrangement allows not only in-axis action, but also moving parts that possess a low mass, keeping shaking to a minimum.

Actually, thousands of rounds are needed to reduce an AR-15 in this condition, however it's renown that the system is much dirtier than most other self loading assault rifles
Actually, thousands of rounds are needed to reduce an AR-15 in this condition, however it's renown that the system is much dirtier than most other self loading assault rifles

It has its drawbacks, though, the main one being that, as it’s been saucily put by many servicemen, “it shits where it eats”: even if most of the gases from powder combustion are exhausted outside the rifle, before venting out of the BCG they all pass directly into the gun action, where dirt can clog things up and cause jams. 

Exploded view of an AR-15 rifle: the three gas sealing C rings in the drawing (between the bolt and BCG in the drawing) are not different from the gas seals on the pistons in the internal combustion engine of a car
Exploded view of an AR-15 rifle: the three gas sealing C rings in the drawing (between the bolt and BCG in the drawing) are not different from the gas seals on the pistons in the internal combustion engine of a car


The Piston operated AR-15


Recently a new breed of AR-15 came to rise: piston operated rifles. A short stroke piston replaces the gas tube, and operates against a bolt carrier appendix much similar to the gas key.

Three versions of a short stroke Piston operated AR-15: the Heckler & Koch MR223
Three versions of a short stroke Piston operated AR-15: the Heckler & Koch MR223
TheHK MR223, field disassembled. Note the op-rod that replaces the gas tube with piston and cylinder integrated in the FSB
TheHK MR223, field disassembled. Note the op-rod that replaces the gas tube with piston and cylinder integrated in the FSB

Original AR-15 are deemed “direct gas impingement”, while the new ones are deemed “piston operated”.  Actually, the old mechanism is not a direct gas impingement mechanism. In a direct gas impingement system (such as the Ljungman Ag m/42) the gases act directly against the bolt carrier, whereas, as we said, here they act into a chamber with a piston. 

Truth is, all AR-15 are “piston operated”, the difference being only where the piston is placed.  

We’ll see the details of the piston gas system (and its own drawbacks) in a forthcoming article. 

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