Chamber pressure (firearms) is the pressure exerted by a cartridge case's outside walls on the inside of a firearm's chamber when the cartridge is fired. It is generally expressed in PSI (pounds per square inch) or copper units of pressure.
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. This pressure in turn pushes the bullet out of the case mouth and into the barrel. During this explosion, the brass walls of the cartridge expand and seal against the inner walls of the chamber. This expansion is what creates chamber pressure, or the amount of force applied to the inside of the chamber by the case. For all commercially available cartridges, the Sporting Arms and Ammunition Manufacturer's Institute (SAAMI) publish the maximum safe chamber pressures.
There are 3 basic methods for measuring chamber pressure:Using a copper crusher
The Piezo method
Attaching a strain gauge to the barrel forward of the chamber
Through the mid-1960s, the most common way of measuring pressure was drilling a hole through the chamber of the barrel and inserting a copper slug that fit flush with the chamber walls. When a cartridge is fired, it compresses the copper slug. It is then removed, measured, and compared to the original dimensions. Using the known properties of copper, the chamber pressure can then be calculated and expressed in copper units of pressure (CUP). While there are now more accurate methods of measuring chamber pressure, the copper crusher method is still used for verification purposes.
Developed in the late 1960s the most common method of accurately measuring chamber pressure is the Piezo method. It is similar to the copper crusher method where a hole is drilled into the chamber, but rather than a copper slug a quartz crystal transducer is inserted and attached to sensitive measuring equipment. 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.
The strain gauge method is the least accurate method of measuring chamber pressure using equipment, but has the advantage of being the least expensive and requires no permanent modifications to the firearm. A strain gauge is attached to the barrel just forward of the chamber. Upon firing, the barrel stretches briefly, and this stretch is measured by the gauge. 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.
The force that is not exerted on the chamber walls is used to push the bullet down the barrel. Because the volume of the case is relatively small, the pressure closest to the chamber will be higher than at any other point in the barrel. Compared to the temperature of the powder being burned, a relatively small amount of energy and heat is transferred from the powder in the barrel to the barrel walls. Therefore, the entire process can be considered an adiabatic process, or no heat is lost during the rapid expansion of the gases. Thus, the ideal gas law can be used to express the difference in pressure as the bullet travels along the barrel:
where:P1 is the initial pressure
P2 is the pressure acting against the tail of the bullet at a given point
V1 is the internal case capacity, or volume of the loaded cartridge
V2 is the internal case capacity plus the volume of the barrel from the chamber to the bullet
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, or as the volume of the explosion increases. Likewise, it should be noted that 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. Because of this, the rifling at the throat will erode faster than the rest of the barrel.
Several measures can be taken to decrease the rate of throat erosion due to pressure, some of which can be achieved by handloading.
- If a bullet is seated further out (i.e. closer to the rifling) it will increase the internal volume of the case. By examining the ideal gas law, PV=nRT, it can be seen that by increasing the case volume, the pressure inside the case is reduced. This in turn reduces the chamber pressure and the amount of force exerted upon the throat.
- If the amount of powder is decreased, the explosion inside the case will be smaller and will result in less pressure.