Well a rifle can Instant kill a Rabbit, Deer or Wolf in one shot. But even the bear can survive a headshot, for a bear to die instantly from a headshot it would need to be a critical hit, criticals being a chance sort of thing. However, a bear would die from a eadshot due to bleeding, it would die approximately 1 hour after being shot in the head by a rifle due to blood loss. I can't say the same about the moose for sure but I would imagine it works the same.