Its principle is that there is a device called a grapple at the top of the rifle bolt. When the bolt re-enters, the hook hooks the flange at the bottom of the shell under the action of the spring, and there is a protruding device called the ejection tappet inside the shell, which drives the shell to retreat together when the bolt retreats. When it reaches the position of the withdrawal tappet, the withdrawal tappet leans against one side of the shell, so that the shell is separated from the hook. At the same time, the forward pulling force of the bolt is inconsistent with the lateral resistance of the pulling tappet, so the shell casing.
No matter what kind of gun, its shell withdrawing device and principle are basically the same.