I think if I can get the barrel 3 inches in my mouth then the muzzle would be 3 inches from brainstem . The muzzle blast would get the brain stem, the pressure wave as well as the buckshot
if you hit the brain it's painless. it takes many microseconds for the brain to process what is going on . the brain is living in the past. research that on the internet. the speed of 1600 feet per second buckshot is about 1/2 kilometer in 1 second . think about how fast that is and how fast it will travel 24 inches to destroy the brain. there is no way . it's like the ending of the sopranos . won't hear the shot if hit the brain if hit only the face then yes pain.
It takes time for information from our eyes to reach our brains and become part of our conscious experience. So our brains use predictions to make up the delay.
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A high velocity bullet from a rifle is even faster going 3200 feet per second , almost 1 kilometer per second . try to visualize how fast that is and why imo nothing can live through that if hit the lower brain areas .