In his Zombie Survival Guide, Max Brooks dismisses the much-loved shotgun as a survival weapon, because its ammo is so big and heavy, but Pat Kilbane counters that with his own point about shotgun hit probability:
If you are shooting at a zombie with a single projectile, your point of aim will have to fall within the 6-inch circle corresponding to the frontal area of his brain (the average human brain is about 6 inches wide extending from the top of the head to just below the nose). That’s a small target to hit, especially when you consider that it will be jerking and swaying as a result of the zombie’s uneven gait.
Exacerbating the problem is the fact that you will be in a horrifying situation, which will likely induce a massive adrenaline dump (and perhaps another type of dump as well). With your heart rate soaring over 150 BPM, your fine motor skills won’t be what they were at the practice range
Circle A The target, a zombie head — brain, really — is roughly six inches across, or three inches in radius.
Circle B At 10 yards, a buckshot load has spread out into a disk roughly 10 inches across, or five inches in radius.
Circle C The lethal point of aim then is anywhere within eight inches of the center of the target.
Discussing zombies makes this all fanciful and palatable, but the real-world British experience in Malaya corroborates exactly this point of combat shotguns hitting more often than other weapons at close range — twice as often as rifles.