What is the action of the gun when the bullet is fired from the weapon?

When a bullet is fired from a firearm, the action that occurs is the ignition of the primer, which in turn ignites the gunpowder contained in the cartridge. This process begins when the trigger is pulled, which causes the firing pin to strike the primer. The primer contains a special compound that, upon impact, creates a small explosion. This explosion ignites the gunpowder, generating rapidly expanding gases that ultimately propel the bullet down the barrel at a high speed. Thus, the ignition of the gunpowder is crucial to the firing process and illustrates the basic operation of how ammunition functions within a firearm.

In contrast, while the barrel does indeed heat up due to the combustion of the gunpowder, this is a secondary effect rather than the primary action. The magazine releasing another round and the trigger mechanism resetting usually occur after the firing process and are part of the cycling of semi-automatic or automatic firearms, rather than the immediate action of firing the bullet.