It is a normal manifestation that takes place in a highly stressful situation and a reaction to adrenaline. Your body goes through rapid temporary changes is is actually auditory exclusion not tunnel hearing.
With the adrenaline dump your body gets ready for fight or flight, you lose fine motor skills, your hearing seems to be diminished, the world appears to slow down. Blood is channeled away from the surface to your core your brain is in hyper drive absorbing everything, which is why everything seems like it is in slow motion. You can and should train to break tunnel vision, that is partially why you should train for the 360 scan. Coming down from a severe adrenaline dump you may be nauseous and you start shaking everywhere.
There have been numerous studies.
My rifle and pistol are tools, I am the weapon.