The Prcincipes of Thermodynamics are very imporantt when i come to designing firearms because they govern how well the entire gun as an engire work.                            

                                    PV=nRT

      
    The ideal gas law is what governs how expanding gases inside of a weapons behave. As stated it says that the pressure multiplied by the Volume is equal to the temperture Multiplied by the the number of moles of the material and the universial gas constant. In the cas of fire arms black powder is ignited, releaseing a very hot gas. This hot expanding gas is what operates the engine inside of the weapon.

       The above picture is a very basic heat engine. The Hot Source of the engine is the expanding gas, while the cold sink in this system is the area around the gun. After the weapon finishes its cycle, and loads another round into the firing chamber, all of the gas freely vents out of the gun.