Powder burns down the barrel and adds to pressure. After all the powder is consumed no more is added. .. I suppose you can run your truck on empty too. It is all the same, you need fuel.
If you run a truck while fueled at 90 mph, and then it runs out of gas, the truck briefly will be able to push something in front of it forward (e.g., slamming into Prius) even though the gas previously has been burned off. It's the moving truck that matters, not its engine that made it move in the first place.
