Eugene Stoner is credited as the design engineer who created the modern day musket– the fascinating machine known as the AR-15. At the heart of Stoner’s design is the direct gas impingement mechanism. As a shot breaks this is what happens:
Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle’s front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base into the AR-15’s upper receiver. Here, the gas tube protrudes into a “gas key” (bolt carrier key) which accepts the gas and funnels it into the bolt carrier.
What you see in the image above is more than the bolt carrier group (the bolt, firing pin and bolt carrier)- the grey part in the middle of the diagram is the bolt carrier and the light blue portion is the bolt. The firing pin is shown in purple.
The gas pushes the bolt carrier group backwards like a piston. As the carrier is driven backward by the gas, the bolt is pushed forward by the gas- unlocking it from the lugs on the chamber. As the bolt carrier continues to travel backward, it rotates the bolt slightly. The fired (and contracted) cartridge case is pulled from the chamber. As the cartridge case reaches the ejection port, the case pivots on the extractor hook from pressure of the ejector until it is sent flying free of the rifle- ideally leaving a lovely brass mark on the shell deflector.
The bolt carrier continues backward into the buffer tube (compressing the buffer and operating spring) while re-cocking the hammer until operating spring pressure or the buffer stops it. The operating spring returns the bolt carrier forward where it strips another round from the magazine up the feed ramps and into the chamber.
As the cartridge stops in the chamber, the bolt continues forward, causing the extractor to snap over the rim of the cartridge case. The bolt finally stops against the case head, but the carrier continues forward- where the cam surfaces, rotating back into a locked position when it meets the lugs on the chamber (thanks to Randall_Rausch for helping with this description).
It’s interesting to note that the bolt rotation turns the locking lugs automatically, the same way the bolt handle is turned manually on most bolt-action rifles. The lugs ensure the cartridge’s position in the chamber is correct and that the pressure is sent down the barrel, pushing the bullet. What a magnificent design, Mr. Stoner!