Pressure is a force that is exerted in all directions in an enclosed area (the chamber being most important - hence the chamber on a firearm is thicker than the barrel needs to be). As the volume of space (down the barrel) increases from the shot load moving at its velocity [in one direction], the pressure is lessening all around the barrel and continues to dissipate until it is released by the shot load exiting the barrel. If the pressure is too great for the barrel's wall thickness {think over-bored after factory specs}, a failure can occur as can happen if the muzzle is plugged. Disaster most likely.
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