There only appear to be two storm troopers in the 3DW ...and only one carrying a gun. So i assume that's the one you mean. Yes, the entire figure is grouped, so unless you enter Group Edit Mode, the whole thing will rotate. Inside that group, all the limbs are sub-grouped. You need to double-click the outermost grouping in order to get down to the next level, then apply the Rotate and Move tools to those sub-groups.