How does an m4 assault rifle work?

You pull the trigger. The hammer strikes the primer. The primer ignites the powder. The ignition of the powder creates rapidly expanded gases which separate the projectile from the cartridge. These gases follow the projectile through the barrel. When these gases reach the opening, they travel up the gas tube and put pressure against the bolt carrier. The bolt carrier is pushed back. As this happens, the extractor grabs the spent casing, and it is pulled out as the bolt carrier is forced back and ejected. The bolt carrier travels to the rear of the rifle, into the extension tube. Its movement is stopped by the buffer. At that point, the return spring - which was pushed on and made tense by the travel of the bolt carrier) pushes forward, returning the bolt carrier to its original position. As the bolt carrier travels forward, it catches the rim of the next cartridge, and pushes it into the chamber. The bolt face enters the chamber behind the cartridge and rotates until it locks into the chamber as the bolt carrier is pushed forward.