 As the bullet is being propelled down the barrel, it is pushed by high-pressure gases which are formed due to the combustion of the propellant contained in the metallic case.When the bullet first starts to move, the pressure in the cartridge case and barrel rises to more than .As the bullet moves down the barrel, the pressures begin to dissipate due to the greater volume of the space from the shell casing to the bullet as it travels through the barrel, and then drops precipitously as the bullet leaves the barrel, which causes the gases to expand very rapidly in the atmosphere since they are now no longer pressurized.