I am quoting myself from an earlier thread. I wrote this awhile ago showing the various ways pressure is measured in a firearm by SAAMI, CIP and NATO. Thought it might be helpful to this thread also.
SAAMI and CIP use very different methods to measure the pressure. Mega rambling warning.
SAAMI pressure testing for the current Transducer method uses a piezo force sensor (from PCB Piezotronics) attached to a conformal post of a precise diameter that extends through a matching hole the pressure test barrel and touches the outside of the cartridge case (thus nearly every cartridges has to have a cartridge specific sensor assembly, though some family of cartridges share sensors (ie 243 Win, 260 Rem, 308 Win ). It is typically located forward of the middle of the case but behind the shoulder if the cartridge has one. When fired the pressure pushes on the case wall that pushes on this force sensor. Pressure is inferred from the force measurement. A calibration method/apparatus exists to correlate actual chamber pressure to the force and compensate for the brass thickness over the sensor post. But because this measurement is made through the case wall you have to use new brass and the brass has to all from the same batch both your empty calibration cartridge for calibration and your actual test lot of ammo. SAAMI pressure testing on reloaded brass introduces a lot more error due to variation in residual brass hardness from previous firings. The advantage of the SAAMI method is the sensor is well protected and after calibration testing proceed fairly quickly without special holed ammunition or consumable crushers. The US military method is based directly on the SAAMI method and uses the same hardware.
The much older SAAMI Crusher method is located in a similar location on the side of the case but uses a cartridge with a hole in it (thin tap applied to retain powder). The hole in the case has to be aligned to a hole in the test barrel. This allows allows pressure to push on a gas check (gas seal) that pushes on an piston that acts on the crusher (a cylindrical piece of very pure annealed copper or lead made to very precise dimensions). Pressure is then inferred from how much crush (change in length) is measured in the crusher. Each batch of crushers comes with a calibration chart that correlates changes in length to a peak pressure. This method is still in use by a surprising number of companies but it is moderately expensive (due to consumable crushers) and very slow and tedious to conduct.
The CIP method is similar to crusher in that it uses a cartridge with a hole drilled in it at a specified location. The hole is aligned with a port in the pressure barrel but unlike the crusher method the gases act directly on a piezo pressure sensor (Kistler) and unlike SAAMI CIP measures pressure directly. In the picture below the CIP sensor would be the left one. CIP has the advantage of probably being the most true chamber pressure vs time data curve. The problem with CIP is it also takes special holed cartridges. It does change initial case volume due to space between the hole in the cartridge and the actual sensor diaphrame potential reducing peak pressure very slightly Since the sensor is directly expose to hot propellant gases, sensor life is shorter than the SAAMI method The NATO EPVAT method uses a similar direct pressure sensor like CIP but it located at the case mouth as shown by the right sensor in the picture below. This has the advantage of not needed holed ammunition but it misses the the earlier part of the pressure curve that happened before the bullet moves past the sensor location exposing the sensor to the gases.
Measuring pressures that are this high and this transient is very difficult and the method still has far more effect on the measurement than most people what to admit. I know some US ammo companies that despite the SAAMI transducer method being faster, cheaper and easier to do still use the Crusher method on ammo if the ammo predates the acceptance of the transducer method. Not because the Crusher method is better, it's not, with many short comings, but it is very repeatable and producing consistent ammo is more important than going to a new measurement method.