Stuns are only available for Enforcers and are unlocked when you reached Cop Role Level 4. Stun grenades play in a very small niche, causing lots of stamina damage, and minimal health damage to aid Enforcers in arresting Criminals.
“By lowering the amount of explosives in the package, Armas has created a version of the standard grenade that is lighter, allowing the user to carry more and throw them further. Careful though, it also comes with a reduced priming fuse.”
It is recommended to cook this grenade to surprise your enemies. To cook a grenade, you simply hold down the grenade button for a certain amount of time before letting it go. If you hold it for too long, it will explode in your hands.