People are sometimes mystified as to why the Oerlikon cartridge has a rebated head, and also why fired cases have the neck blown out.
Here is a photo of a neat wooden (training?) model showing a sectioned barrel with dummy cartridges and bolt assembly. It is marked to show the position of the bolt relative to the barrel at start of firing pin movement, at point of ignition, and start of recoil.
(Photo from Amoskeag auction ending 3/30/3030, item 3857)