When a Marine is in overwatch he shoots one shot at any geanstealer he sees perform an action. Tow questions actually. If you shoot at the same stealer more than once you kill it on 5s, I'm pretty sure that is right. This is the one I'm not to sure on. When your marine witnesses a stealer perform the action of attacking if he shoots and kills it does the close combat happen or does the stealer die before the combat?