Pepper gun

The Pepper Gun is a riot control weapon used by security forces. It can stun a target for a few seconds without causing long-term damage. It does not affect MJ-12 Commandos nor monsters.

The Pepper Gun is first found near the front of the Statue of Liberty on the first mission. If not obtained there, it can be found in the MJ-12 base in Hong Kong.

Ironically, UNATCO, a so-called peace-keeping organization, almost never issues Pepper Guns.

Tactics
In a pacifist run, Pepper Spray is useful if you need to get a backshot on a target, but have already alerted him. It's also useful if you're running low on Mini-Crossbow and/or Riot Prod ammo and need to run away.

In a non-pacifist run, it's only use is if you need a fast target to stop moving long enough so that you can headshot him with a Shotgun. Otherwise, dump it.