What force propels the bullet out of the muzzle once the powder ignites?

The bullet is propelled out of the muzzle by the expanding gases created from the burning powder within the cartridge. When the powder ignites, a rapid chemical reaction occurs, producing a large volume of hot gas that expands quickly. This expanding gas exerts considerable pressure on the bullet, pushing it down the barrel and ultimately out of the firearm.

The other options, while related to firearm operation, do not accurately describe the mechanism that propels the bullet. The bolt action refers to the mechanism that loads a cartridge into the chamber but does not generate the force needed to propel the bullet. Gravity influences the bullet's trajectory after it leaves the muzzle but does not affect its initial propulsion. The force of the trigger pull activates the firing mechanism, but it is not the direct force that pushes the bullet out; that energy comes from the expanding gases resulting from the ignited powder.