An average male has about 6 litres of blood and his internal carotid artery clears about a quarter of a litre per minute at rest to supply the brain. When in a stressful situation, like, for example, being shot, that output can double. If we need to lose about 20% of our blood to lose consciousness, our notional male could black out in just over two minutes just through having damage to his carotid. However, that’s two minutes of waiting if he’s not been knocked unconscious by the impact.
But if we’re thinking about brain damage, the extent depends on a whole range of ballistic factors – the velocity, shape, size and make-up of the bullet being key. As it turns out, the brain needs special consideration, not least because it is encased in the skull. One of the first things to consider is that the skull can fracture and how the fragments themselves can become missiles. In 42 cases of civilian gunshot wounds to the brain two neurosurgeons were able to find bone chips in 16 patients’ brains simply by “digital palpation” – which is a complicated medical term for sticking your fingers in and wiggling them about. In other words, a shot to one part of the head may have knock-on effects purely due to skull shattering.
However, the skull also sets up a unique target due to its enclosed nature. If someone gets shot in the leg the pressure of the impact can be released into the surroundings. If a bullet gets into the brain the options are fewer because the pressure waves and, indeed, the brain, are largely trapped inside a solid box of bone. If you want to get an idea of the sorts of pressures involved, just catch a video or two of bullets being fired into ballistic gel and think what would happen if the gel was trapped inside a personally important life-sustaining box."
The whole post’s good reference for anyone who might need to write gunshot wounds to the head.