There is no clear mechanic for it because D&D is a heroic game. They didn't want to put in any rule that could reduce someone with a lot of hitpoints to helplessness in one shot or very few shots. Because, although you may want something like this to knockout NPCS, if it was in the game, DMs would start using it on players.
"Oh ya, you may be a 20th level barbarian, but I hit you in the head with my knockout feat. Now that you are unconscious, I CDG you and you are dead."
Majoru Oakheart