Alot of factors would come into play, I would guess. Each situation would be different I would assume. First of all, even if someone did have brass knuckles on, they would have to actually land the punch for it to work. And it would have to hit a good target. The knuckles may slow down the swing a bit, and an experienced fighter may counter it and put the knuckle-wearer away. I'm sure more opinions will come in on this