Chamber pressures, cartridge cases

The burning of the propellant very rapidly produces a large volume of gases in a confined space accompanied by a substantial temperature and pressure rise. The resultant gas pressure forces the bullet away from the cartridge case and down the barrel of the firearm. The temperature and pressure rise also serves to cause the cartridge case to expand in the chamber, thereby effectively sealing the chamber to prevent any rearward escape of gas (obturation), which would lead to a reduction in pressure and consequently a reduction in bullet velocity. 

When a round of ammunition is discharged in a firearm, the internal gas pressure, and to a much lesser extent the temperature rise, causes the cartridge case to expand tightly against the chamber walls (obturation). This is an extremely important function of the cartridge case as this prevents the rearward escape of gas. Such an escape of gas would reduce the velocity of the projectile and consequently the efficiency of the firearm and could possibly cause a malfunction in the firearm mechanism. 

This function of cartridge as "obturator is due to the expansion of the case against the chamber wall under the pressure of burning proplnt(Ref 10, p 128)... 

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