The Extrema Ratio TK Shrapnel OG Blue is a strikingly coloured training knife from Extrema Ratio. Training knives like this are used in various martial arts to practise disarmament techniques. These training knives, also called dummy knives, are copies of the sharp versions that Extrema Ratio also produces. The difference is that this bright blue blade, made of anti-corodal (an aluminium alloy), is completely blunt. The handle is the same, though.