Which part of a modern firearm has the same function as the lock on a muzzleloader?

The action of a modern firearm is responsible for the mechanics of loading, firing, and ejecting cartridges. This is similar to the role of the lock on a muzzleloader, which also facilitates the ignition of the powder and the firing of the projectile. While the chamber holds the cartridge in modern firearms, and the trigger initiates the firing process, it is the action that encompasses the entire mechanism of operation, including the system that allows a round to be chambered and fired. The stock serves as the structural basis for holding and aiming the firearm, but it does not play a role in the firing mechanism itself. Therefore, the action directly correlates with the function of the lock in terms of operating the firearm.