There are conversion kits (barrel minimum, slide and barrel for some models) that convert the firearm to shoot the Simunition FX rounds (and live rounds won't work). The cartridge is basically a primer powered plastic 7.62mm plastic pellet with a colored non-toxic soap. The conversion barrels don't lock the action, so it becomes a blow-back which is how it can cycle with so little power.
There is no PPQ kit, they do have a P99 kit. Students will use my converted guns (a G19 and 2x 17s) It would be impossible to ever have conversions for the myriad carry guns students would bring. (Or possible with any other simulation system either) They can also use a P226 or 225 if they want, those are my role player guns, but I can give the RP a Glock instead. They can also use a .38/.357 revolver, the conversion for those is just metal rings in the chambers so live rounds can't chamber.
Are Sims sold to civilians?
Another safety feature, see the chamber cut out? If you somehow managed to get a live round to chamber (banged on the slide until the cartridge deformed enough I guess) and it fired...the .355 bullet can't fit down the .30 bore and all the pressure will blow out the hole in the chamber.