Blow-Back is an action that helps to cycle a weapons ammunition by using the gases (or high pressure air) available to move the components that make up the system.

Air or gas acts on the bolt directly or indirectly, pushing it backwards before or after the BB is fired. Depending on the type of loading function, gas may fire the BB before the bolt is moved forward, or after when the bolt is at the rear. Most blowback mechanisms in home made air guns generally fire the BB with the bolt forward relying on the momentum of the bolt under pressure to lock back on the sear.