Many enthusiasts like to build AR-15 rifles and it is important to know exactly how the firearm works. We are going to briefly cover a simplified description of how the BCG functions.
1 When the trigger is pulled, the hammer hits the firing pin sending it traveling forward through a hole in the bolt assembly to strike a primer.
2 When the primer detonates and ignites the powder, the expanding gas propels the bullet down the barrel. As the bullet passes the gas port, some of the expanding gas travels through the gasblock into the gas tube.
3 From here it flows into the gas key which directs the gas into the expansion chamber in the carrier.
4 The expanding gas exerts pressure on the gas rings on the bolt assembly forcing the assembly to move rearward.
5 As the assembly travels, the cam pin forces the bolt to rotate 15° and unlock from the chamber.
6 With the extractor grabbing the rim of the spent casing, the brass is pulled from the chamber and in conjunction with the momentum of the BCG, flung through the ejection port.
7 Gas is also vented through 2 holes on the side of the BCG. The moving carrier cocks the hammer preparing the fire control group for the next shot.
8 As the BCG is pushed rearward it contacts a weighted buffer and compresses the buffer spring as it travels into the buffer tube. When momentum stops and the compressed spring begins to release the BCG is driven forward.
9 If the magazine is empty, the BCG will be engaged by the bolt catch and stop forward motion to allow for a magazine change. If the magazine is not empty, the bolt head will push a bullet up a feed ramp into the chamber.
10 When the round is chambered, the iconic looking bolt head is rotated 15° and locked into the barrel extension. This enables the bolt to withstand the extreme pressures produced by firing a round.
11 The process repeats after the next pull of the trigger.
Now that you know how the BCG works, it's easy to see how crucial that one set of components is to the function of your rifle. The U.S. Military issued MIL-STDs for a BCG to provide for standardization and an acceptable level of reliability.
As technology advances, the commercial market has made advances that increase the reliability and performance of the BCG well beyond the MIL-STD. The reality today is that not all BCGs are created equal and which one you choose for your rifle is subject to a variety of factors.