CUP pressure, expressed in pounds per square inch, is a pressure measurement which has been taken using a particular tool known as a copper crush gauge (or crusher gauge). CUP stands for Copper Units of Pressure. It is specified because measurements taken using this method, while internally consistent, can differ significantly from measurements taken using other methods such as Lead Units of Pressure, or piezoelectric gauges.
CUP pressure is most commonly used in the process of proofing firearms. To determine that a firearm is safe to use with its specified ammunition load, it is 'proofed' by being fired with a proofing cartridge. This is a specially constructed cartridge which is generally between 20% and 40% more powerful than the ammunition the gun is specified for, and has been modified with a hole cut in the base of the cartridge. A crusher gauge is attached to this hole, and a precisely manufactured cylinder of copper is placed in the gauge. When the gun is fired, the pressure inside the chamber forces the gauge outwards and crushes the copper cylinder. By comparing the crush distance to recorded values for known pressures, an indication of the pressure inside the gun chamber can be obtained. This method has been around for many years. Since the measurement is one of pressure, psi (or pascal, newton/meter in SI units) is used. For example, an M16 or AR-15 rifle firing standard 5.56x45mm NATO rounds develops approximately 52,000 CUP pressure psi. For low-pressure applications such as shotgun and small pistol ammunition, Lead Units of Pressure measurements are used as lead is softer and deforms more easily than copper.
Modern measurements using piezoelectric instruments can be more accurate; however, since the readings differ significantly between the two methods at the high and low end of the scale, it is important to identify which method was used to proof a particular gun so that the user knows which ammunition measurement to match to it.