Rigid Bodies.

You may notice that if you add a barrel as a static mesh to your level and shoot it in game mode, nothing will happen. That is because it is static. To add physics to it you need to add it as a Rigid Body. To do that, select the object on the content browser, right click on where you want to place it and add it as a Rigid Body. The new object will react to your shots, giving it more realism.

Rigid body barrels