How an AR-15 Works

Pressing the magazine release frees the magazine from the lower receiver. Rounds are loaded and held under high force of the magazine spring. It's then inserted into the magwell and secured by the magazine catch. Pulling and releasing the charging handle chambers the first round. The charging handle forces the bolt carrier group to the rear. As it comes back, the action spring is compressed. As the carrier returns forward, the bolt strips the top cartridge from the magazine and guides it into the chamber. Housed within the bolt carrier is the bolt. The bolt contains several locking lugs that engage with the barrel extension lugs. As it makes contact with the barrel breach face, the bolt turns and locks into place. The locking function is due to the cam pin housed within the bolt. The pin rides along a track in the bolt carrier, allowing it to rotate. When the bolt is open, it's locked in the forward position and can only rotate once the cam pin has cleared the recess in the upper receiver. In the rare occurrence where the bolt fails to fully close, the forward assist may be used. Pressing the forward assist plunger allows the pawl to engage with the notches cut into the bolt carrier. This will force the carrier forward until the bolt is fully locked. With the selector taken off to safe position, the AR-15 is ready to fire. A recess cut into the selector permits trigger movement. As the trigger is pulled, the hammer is released from the sear. The hammer spring drives the hammer forward to strike the firing pin, causing it to travel forward and impact the cartridge primer. Within the primer is the ignition compound and an anvil. As the firing pin strikes the primer, a spark is created, igniting the propellant inside the cartridge. The expanding gases propel the bullet down the barrel, where rifling grooves impart stabilizing spin on the bullet. The gases escape through a small port in the barrel. Through the gas tube and into the bolt carrier key, forcing it to the rear. The extractor grips the rim of the spent cartridge case and holds it against the bolt face until the ejector forces it through the ejection port, striking the deflector as it exits. As the bolt carrier comes back, it returns the hammer to its cocked position and stays held back by the disconnector. When the buffer reaches the back of the receiver extension, the action spring returns the bolt carrier forward. As it returns, the bolt strips a new cartridge from the magazine and directs it into the chamber. Simultaneously, the extractor clips into the rim of the new cartridge. Releasing the trigger releases the hammer from the disconnector, and again the trigger sear assumes control of the hammer, readying the rifle for another shot. Military variants of the AR-15, such as the M16 and M4, add a third option for fully automatic fire. The inclusion of an auto sear and a hook on the back of the hammer allow continuous operation while the trigger is depressed. Moving the selector to auto, the disconnector is disabled from moving and will be unable to hold the hammer when fired. The auto sear is now able to rotate into a recess in the selector. After a shot is taken, the auto sear rotates forward as the bolt carrier comes back, taking control of the hammer. As the bolt carrier returns forward, the back of the carrier trips the auto sear, releasing the hammer just after the bolt closes. Releasing the trigger allows the sear to again assume control of the hammer. In order to conserve ammunition and promote greater accuracy, a burst mode was later introduced. With the auto sear still present, the disconnector is split in two. The left disconnector takes control of the hammer in semi-fire mode. Switching to burst fire disables the left disconnector. The burst cam and clutch spring allow the M4 to fire up to three rounds at a time. The clutch spring expands to rotate the cam only when the hammer moves in reverse. As the trigger rotates forward, the burst disconnector engages the deep notch of the cam. The cam rotates as the hammer is brought back and the disconnector engages the first small notch. This isn't enough for the disconnector to hold the hammer and the auto sear is tripped to fire another round. The cam rotates again and the cycle is repeated. The cam rotates once more and the hook engages the third deeper notch. The disconnector is now able to reach the hammer, holding it back. The trigger must be released and pulled again to fire another three rounds. Once the final round of ammunition has been fired, the magazine follower pushes up on the bolt catch, holding it open as it returns forward. Inserting a new magazine and pressing the bolt catch will chamber a new round. Using a cartridge, the front sight post can be adjusted up or down for zeroing. The rear sight includes two apertures. For short ranges, the 0 to 200 meter is used. For longer ranges, the smaller aperture is used. Turning the elevation knob will raise and lower the sight depending on the distance. The knob on the right of the sight is used to adjust for windage. The calibration lines on the back of the sight help control windage adjustments during firing. The carry handle on the AR-15 can be removed to allow for alternative optics such as holographic sights or longer range scopes like the ACOG. Many ARs are equipped with an adjustable stock. The stock rides along the receiver extension, commonly referred to as the buffer tube. Pressing the release lever, moving it to the desired position, and releasing it will lock it in place. The ejection port cover can be closed when the rifle is not in use to protect the inner parts from outside elements. The cover opens automatically when the rifle is fired.