[citation needed] When the firing pin in a firearm strikes the primer, it ignites the powder inside the case, creating an explosion that generates a large amount of pressure, often exceeding 50,000 PSI (344.7 MPa).
Maximum safe chamber pressures for commercially available cartridges are published by bodies such as SAAMI, CIP and NATO.
[4] This method generally yields more accurate readings than the copper crusher, and is more cost efficient due to the fact that the transducer can be reused.
[5] This method is generally reserved as a way of relatively comparing different cartridge loads as the strain gauge reading is not as accurate as a copper crusher or Piezo test.
Thus, the ideal gas law can be used to express the difference in pressure as the bullet travels along the barrel:[6] P1(V1)κ=P2(V2)κ or P2=P1(V1/V2)κ where: Looking at this thermodynamic equation, it can be seen that the amount of pressure acting upon the bullet decreases at it travels down the barrel due to the increase of the gas volume.
[6] Likewise, the part of the barrel that is subject to the highest pressures is the throat, or the point closest to the chamber at which the bullet engages the rifling.