The BCG or Bolt Carrier Group is a “group” of parts that work together to load a cartridge into the chamber (battery), extract gas from the barrel (through the gas system), extract, eject and repeat.

The BCG holds the bolt and allows it to rotate with the cam pin. The bolt holds the firing pin, the extractor, the ejector and has gas rings on it to seal it within the BCG. The BCG has a gas key mounted on top of that engages the gas tube and sends gasses into the BCG allowing the bolt to rotate out of battery and driving the BCG back and into the buffer and spring that is in the buffer tube (stock). The BCG also re-cocks the hammer in the lower.

Bolt carrier groups are available in many finishes. Standard chromoly, chrome, nickel boron, nitride or melonite are available or combinations of these finishes. Some say this is just personal preference but some finishes slide better and last longer than others. Nickel boron is slipperier than standard chromoly and may wear better. Some like the harder surface finish of the nitride finish.

Next Part 6 - The Changing Handle