Which major component group consists of the ejection port cover and the housing of the bolt carrier key?

The major component group that includes the ejection port cover and the housing of the bolt carrier key is the Upper Receiver Assembly. This assembly is essentially the top portion of the rifle and contains critical components necessary for the operation of the firearm, including the barrel, bolt carrier group, and ejection port cover.

The ejection port cover serves to protect the chamber from debris and moisture when the firearm is not in use, ensuring reliability in various environments. The housing of the bolt carrier key, which is part of the gas system, plays a crucial role in the operation of the rifle by directing gas from the fired cartridge back into the bolt carrier, allowing it to cycle and chamber the next round.

Understanding the function and components of the Upper Receiver Assembly is essential for maintenance and troubleshooting, making it a foundational aspect of the M16A3/M4A1 service rifle's design and operation.