>>4891475
The Sterling (and also the Sten, which it's vaguely based on), operates on a principle called open-bolt, which means that when the gun is ready fire, the bolt is locked back and the ejection port is open, and as soon as the bolt closes, it will immediately fire whatever cartridge is in the chamber.
Old subguns like the Sten, Sterling, Greasegun, Uzi and MP40, all work in this basic principle, and it's really fascinating just how mechanically simple these guns are; not counting the magazine, the only moving part in the gun when cycling is the bolt itself, which is essentially just a solid weight, since there's no kind of gas piston, or delaying or locking mechanism. This also made these guns quite cheap.
When you pull the bolt all the way back, the sear catch grabs it and holds it in place, compressing the recoil spring, then when you pull the trigger, the sear releases the bolt, and the recoil spring shoves the bolt forward with haste, where it picks a cartridge from a magazine, puts it in the chamber, and the inertia of the bolt's movement goes through the fixed firing pin and fires the cartridge immediately as it's in the chamber.
Then, the propellant gasses of the fired cartridge pushes back against the bolt, and it goes flying back, to either be caught by the sear, or if you're still holding the trigger, bounce on the recoil spring and continue the firing cycle, loading the next cartridge and immediately firing it, and so on.