In a semi-automatic firearm, whether
it’s a pistol, rifle, or shotgun, when the round is fired, the
expanding gas from the cartridge launches the projectile and then
opens the chamber, ejecting the spent cartridge and, if available,
using a spring system to slide the next cartridge into place ready
for the next trigger pull.
The gun will not fire the next round and most guns are designed to
prevent this from happening. This means one trigger pull for each
shot.