[dubious – discuss] Pressure is a fundamental physical parameter that is defined as force divided by area.
A chamber pressure measured with a copper crusher gauge would therefore be expressed in MPa (CUP) in the ISU.
With the availability of inexpensive, reliable transducers since the 1960s for actually making chamber pressure measurements, the industry almost universally has begun to move away from crusher guns for estimating chamber pressures, towards favouring making actual measurements.
Transducers are also faster to use in practice, as they do not require the careful measuring of the copper or lead cylinders after firing.
Another outcome is that design margins are now better determined, which has the effect of increasing the long-term safety of firing multiple thousands of rounds in a gun.