It wasn't with a gun.
The most amazing shot I ever saw was from my little brother's slingshot.
You know how a kid will get a slingshot and then proceed to just wander around shooting things with it? After a while, it just becomes second nature, and the kid stops thinking about it (I suppose this is where we'd all like to be with our guns) and just does it. It's not super-important whether the kid hits anything, so he's mellow about it.
Anyway, Mike had gotten pretty good just plinking with pebbles, good enough that I was pretty darn impressed. He didn't realize it was any big deal, though. I gave him some 5-gram weights to shoot (Dad was throwing them out from his lab) and was showing off his accuracy to some neighborhood kids ("All right, now hit
that rock.") when a sparrow flew by. I told Mike, "Shoot that!"
He didn't even think, he just swiftly drew back and shot the bird out of the air. Dropped like a stone.
Blew its head off, actually, although that was a fluke - he would never claim that he'd aimed for the head.
Years later I did something similar (but not nearly as impressive to me) with a throw. Walking back and forth from my dorm's cafeteria (in another building), I'd throw my keys at birds congregating in the path. My big leather keyring would catch enough air to slow down the keys enough to where the birds would be able to fly out of the way by the time the keys got there, but I got pretty accurate with the throws. After months of this, I nonchalantly hucked a giant bolt I'd found at a squirrel - and broke its neck. Felt really bad (hadn't really intended to hit it), and had to kill the convulsing thing with my boot.
(Some great stories here, btw.)