Final answer:
When a firearm is shot, only the bullet is propelled out of the firearm, while the spent casing remains inside.
Step-by-step explanation:
When a firearm is shot, the bullet is propelled out of the barrel due to the force generated by the explosion of gunpowder in the cartridge. However, the whole cartridge does not propel out of the firearm.
Inside the firearm, there is a mechanism that contains the cartridge and allows the bullet to move forward while the spent casing, which is the part of the cartridge that holds the gunpowder, stays behind.
So, when a firearm is shot, only the bullet is propelled out of the firearm, while the spent casing remains inside.