It isn't true that much of the brain isn't used. That is an old myth from the early days of localization that lacked the techniques we have today. It's active all over its structures.
However, the structures can be very specialized and unless you disable ones that directly deal with life support as in the hindbrain, you can survive but lose specific functions. You can compensate for some but some leave permanent disabilities.
Damage to the cerebral cortex can be survived - the classic case of Phineas Gage shows someone who took a shot in the head far surpassing a gun round and survived but with severe personality changes.
Children can have an entire hemisphere removed and survive and have fairly normal language.
So a shot to the brain is like saying a shot to the torso - if it doesn't take out a vital function, you still can be in the fight or carry out actions if you don't quit.