if you want the skeleton to roughly face the target i actually modify spine_03, although this does depend on your animations
if its just the weapon i do recommend fixing the socket and then adjusting your weapon mesh so the barrel faces forward, (X axis) just for consistency
if its just for bullets then it doesnt really matter, if you just trace from Muzzle to Target and use the UnitDirection it should be fine
another option is to look into the ControlRig which has an inbuilt Aim function