6 no Trigger

A unique take on the 'assasin-for-hire' story, as the assasin 'Tsubame' actually is four different people with four different modes to kill. Penguin is a small quiet kid who has beastly sniper skills, Washi (Owl) has a huge gattling gun grafted to his shoulder to mow down his enemies for the sake of 'justice', Suzume the cute girl who wields dual pistols, and of course Tsubame himself who prefers huge rifles and martial arts. The four have a mechanical arm which when switched allows one of the others to take control. How did Tsubame end up this way? What is the secret of the arm?