Adrenaline
Adrenaline and endorphins are crazy things. They can enable injured people and animals to do the strangest things. During an attack or fight endorphins numb pain and adrenaline constricts blood vessels, channeling blood to where it is needed most, usually the large skeletal muscles. I have experienced this first hand. Not due to a shooting, but another life and death scenario. It is a totally personal experience, but in my case, I stopped felling pain, got slight tunnel vision, I lost color vision for about a minute and only saw in black and white, time slowed down, and I was ridiculously strong. After the incident, I needed a medic for sure, but I was able to fight off four attackers who were all larger than me, and escape to safety, inflicting a world of hurt upon two of them in the process.
My experience leads me to believe (for the most part) that as long as the brain and spine are not damaged, and the will to fight or flee is there, that someone or something will continue to fight, no matter how badly wounded, until they are either safe or bleed out, or are incapacitated in some other way.