Simunition and Airmunition cartridges are actually 7.8mm (for the 9mm) and the firearm is to be fitted with a kit that makes it impossible to cycle in a lethal projectile. This "condition blue" protocol is supposed to make it idiot proof, until a smarter idiot comes along.

You can imagine what could happen if a lethal projectile got in the mix when you were playing force-on-force , thinking you were firing simunition.

The firearm is also maked as a condition blue dedicated training tool usually with a replacement slide, however it can fairly easily be torn down and returned to normal. If you fire the sim round in a regular pistol; if it fires, it will bulge the cartridge and jam but not likely to damage the firearm.
9mm is the least likely to really hurt if you get hit as it travel at about 250 fps and impact energy is less than 3 joules. 38 and 40 are faster and hurt a little, and sometimes alot more.