What part of a firearm is primarily a metal tube through which the projectile travels?

The correct answer is the barrel, as it is specifically designed as a metal tube that directs the projectile when a firearm is discharged. The barrel is essential for providing stability and accuracy to the shot, allowing the bullet to travel the intended path after being propelled by the explosive reaction within the firearm.

The stock is the part of the firearm that is held by the shooter and provides support, rather than being involved in guiding the projectile. The action refers to the mechanism that loads, fires, and ejects cartridges, encompassing various components that are critical for the firearm's operation, but does not serve the function of guiding the projectile itself. The trigger is the control that the shooter pulls to initiate the firing process, but it does not play a role in the physical pathway of the projectile. Thus, the barrel is the defining feature in this context.