Adrenalin is a powerful drug the body dumps into the bloodstream in "fight or flight" scenarios. Does wonders to help survival until the body can't compensate anymore.
Add recreational drugs to the mix and it's a chemistry experiment!
Keep shooting till they drop is wise advice. The majority of GSW patients I've treated had multiple wounds. Aim dead center in the chest, hit the heart, spine ideally, lungs too.
If that's the invisible man at the door, tell him I can't see him!