Fun fact: the 5.7mm round the P90 uses was not being produced as a blank. So the armorers bought TONS of live 5.7mm and made those into blanks.
This actually caused a supply bottleneck for none other than the US Secret Service, who were also using P90s at the time. They actually had to ask, or buy, not sure which, several thousand rounds from the show armorers for training until ammo production could ramp up stateside IIRC.