How did the German MP18 operate using blowback?

The German MP18 operated using a principle known as simple blowback. In this mechanism, the energy generated by the firing of a 9-mm round creates high-pressure gases that propel the bullet forward while simultaneously blowing the spent cartridge case backward out of the chamber.123

This rearward movement of the cartridge case forces the weapon's heavy bolt back against a spring, which serves two primary functions: it trips the mechanism to eject the spent casing and compresses the spring to store energy for the next cycle. Once the bolt reaches its rearward limit, the compressed spring forces it forward again, feeding a fresh cartridge into the chamber. If the trigger remains depressed, this cycle repeats automatically until the ammunition is exhausted or the trigger is released. To ensure the mechanism did not operate at an excessively high rate, the MP18 utilized a heavy bolt and spring, which limited its rate of fire to approximately 400 rounds per minute.23