Firing Process of an AR-15 Firearm Platform

Introduction of the AR-15 One of the most universal firearm systems ever created is the AR-15 platform. The AR-15 platform is a semi-automatic firearm based on the design of Eugene Stoner’s original AR-10 produced by ArmaLite in the late 1950s. In 1959, the AR-10 was converted from using a 7.62x51mm cartridge to the much smaller 5.56x45mm cartridge and the AR-15 was born. The AR is actually a shortened designation for ArmaLite, the firearm manufacturer which originally designed the firearm. The M-16 used by the United States military is based almost entirely on the AR-15 design. The construction of the AR-15 is mostly aluminum and many different manufactures have a version of their own with many of their parts being compatible with others. The firing process is how a firearm is shot, sending a projectile down range to the designated target.A Colt brand AR-15 is displayed in Figure 1.

Figure 1: Colt AR-15 Hammer Pulling the Trigger Sear Trigger Assembly: 1. Trigger 2. Sear 3. Hammer Sear/Hammer Catches 4. Sear/Hammer Catches Trigger

Figure 2: Trigger Assembly Every firearm has a safety with at least a fire position and a safety position where the gun cannot be fired. Once the safety is changed to the fire position it is ready to be shot. To fire the gun the trigger must be pulled. When cocked, the front sear catches the hammer and holds it in place. One the trigger is pulled the sear slides out of the way of the catch on hammer, releasing the hammer. The hammer then strikes the firing pin, which then strikes the primer of the cartridge. Figure 2 shows the trigger assembly with the hammer midway between cocked and fired position.

Expansion of Gases

Cartridge is fired and hot gases expand from the cartridge. Gases flow down barrel.

Gases enter gas port. Gases enter gas tube. Gases flow through gas tube to gas key.

Gases flow through gas key and into bolt carrier.

Operation of the Bolt Carrier Group Inside the bolt carrier is located the bolt, firing pin, firing pin spring, extractor, and firing pin retaining pin. The bolt has gas rings which create a seal between the bolt and the bolt carrier. As the gases flow into the bolt carrier, they help keep the bolt in place. The motion the gases push the bolt carrier back. On the front of the bolt is some squared off sections extending from outwards, called lugs. These lugs allow the bolt to stay locked in place as the firearm fires, preventing the bolt from violently blowing open as the gases expel out of the cartridge. As the bolt carrier moves away from the bolt it turns the bolt with the help of the cam pin, unlocking the lugs on the bolt. The bolt carrier is pushed back against the buffer spring assembly. The motion of the bolt carrier turns the bolt. Figure 4 shows the parts of the bolt carrier assembly.

Figure 3: Bolt Carrier Group

Extraction of the Spent Round As the bolt pulls backward with the bolt carrier, the extractor spring presses forward on the left side of shell’s rim. As the extractor spring presses forward on the left side, the extractor firmly grips the right side of the shell’s rim, forcing the spent shell to the right and out the ejection port, located just above the trigger. Figure 4 shows the extractor and extractor spring located on the front of the bolt face.

Extractor

Extractor Spring

Figure 4: Bolt Face

Reoperation of the Bolt Carrier The buffer spring assembly pushes the bolt carrier forward. The buffer spring assembly also helps to dissipate a large percentage of recoil. As the bolt carrier is pushed forward the bolt pushes a new cartridge from the magazine into the cartridge. As the bolt proceeds forwards, the cam pin causes the bolt to turn. The backwards motion of the bolt carrier also pushes the hammer back into place, where it is again locked by the sear.

What is the Different between Semi-Auto and Full-Auto The difference between the firing process in a semi-automatic and fully-automatic AR-15 platform is the sear. In a semi-automatic, the trigger can be held in through the firing process and the hammer will still remain locked in place by the sear. In a fully-automatic firearm the sear allows the hammer to be released if the trigger is held in the firing position. The sear will continue to release the hammer until the trigger is removed from the firing position.

Review To review, the process starts with the removal of the safety. The trigger is then pulled, which releases the hammer. The hammer strikes the firing pin which hits the primer in the cartridge, firing the cartridge. The gases from the round being fire are propelled down the length of the barrel. The gases enter the gas block through the gas port. From the gas block, the gases expand and flow down the gas tube into the gas key which is connected to the bolt carrier group. They then enter the bolt carrier group where they push the bolt carrier back and holds the bolt in place. The bolt carrier pushes back into the buffer spring turning the bolt. The bolt carrier also pushes the hammer back into position where it is locked by the sear. The extractor spring presses forward on the left side of the spent case. The extractor holds onto the right side of the rim of the spent shell. The spent shell is then tossed out of the extraction port. The buffer spring then pushes the bolt carrier group forward again. As the bolt carrier proceeds forward the bolt turned again into a locking position. Once the trigger is released and pressed again the process will start over again.