Complex sentence: Semi-automatic firearms, when fired, automatically extract the spent cartridge casing and load the next cartridge into the chamber, ready to fire again; they do not fire automatically like a machine gun; rather, only one round is fired with each trigger pull.
Split into simpler sentences: Semi-automatic firearms, when fired, automatically extract the spent cartridge casing and load the next cartridge into the chamber, ready to fire again. Rather, only one round is fired with each trigger pull.