When you get shot in the chest, your breath rasps as your lungs are not longer sealed and begin to collapse. It's actually quite painful. As blood flows into your punctured lung and you're trying to breath, you begin to hack up/cough up blood as you're literally drowning in it. It becomes worse as people apply pressure on your chest to reseal the lung and begin CPR which is also quite painful. If you're not cracking the sternum for CPR, you're doing it wrong. That amount of force/pressure is also going to eject blood.