ok, the char moves with the A and D keys and you shot with the mouse...
the mouse is a crosshair and the weapons(arms) are always aiming to the mouse
I played a game like this once, it's was called "abuse".
i think it's simple to do, it's the same as a normal shooter game(metal slug like) but with 2 simbols(classes) inside 1
the player has 2 classes in it
the player has the registry point in the bottom
the player.arms too
and the player.body has the registry point where the arms should be fixed(torso);
the arms part position is the same X and Y of the body
the arms part has 2 simbols, one inside of the other
this._x = _parent.body._x;
this._y = _parent.body._y;
the arms itself and the gfx;
then you could just make gfx._yscale *=-1 to make it not get upside down;
it's harder than i tought