What is the primary function of propellant gas in a cannon?

The primary function of propellant gas in a cannon is to push the round out of the chamber. When the propellant within the cannon is ignited, it rapidly generates a significant amount of gas and pressure. This high-pressure gas is what ultimately drives the projectile down the barrel and out of the cannon.

The ideas behind the other functions can help clarify why pushing the round out of the chamber is the primary role. For instance, while the ignition of the projectile is crucial, that process is handled by a separate charge or component within the ammunition and not by the propellant gas itself. Likewise, cooling the chamber isn't a function of the propellant gas - in fact, the combustion process generates heat, which typically requires cooling systems or methods to manage. Stability in aim involves other systems and mechanisms rather than the propellant gases, as aiming is determined mainly by sighting systems and barrel alignment. Thus, the correct answer highlights the decisive role of propellant gas in propulsion, which is its primary purpose in the functioning of a cannon.