Basically what I want is the tranquilizer pistol from the Metal Gear Solid series.
No, it doesn't have to actually put players/bots in an unconscious state, it just has to reflect the stamina damage it inflicts as actual damage.
For instance,
if the bot/player gets a dart to the head, they are instantly out (100 damage).
If they are hit anywhere else, there will be a minor decrease in health (like 20 damage or something) and their health will slowly drop to zero. That way, the player can either continue to shoot to get the job done faster, or run away and wait for the enemy to drop.
There should be a firing delay of about 1 second.
As for appearance and sound, anything resembling a silenced pistol should do (you could even just borrow the model from the silenced default gun add-on if you can find it), and you could probably just take the sounds from that as well.