[@Ollumhammersong] You need more than 3 inches to kill someone [i]quickly,[/i] though. At least if you're aiming for the center of mass. Like, you stab somebody in the chest with a knife a few times, it will likely deal lethal damage, but it's probably not going to instantly slay them, so far as I'm aware. They might not even realise they've been stabbed at first, and could consequently react by stabbing or shooting you back in that case. The equivalent sword attack is one or two swings at their center of mass with the blade - takes more time per attack with less precision, but there's consequently more force behind each attack, meaning you're probably going to lop their arm off or tear open a gaping hole in their abdominal cavity with each one if the attack isn't stopped by armour, and in 40K armour that stops swords most likely stops knives to the same areas.