Rick Raphael's "A Filbert Is a Nut" (short story, humor, free): A lunatic asylum inmate who could make magic
Not exactly the kind of story I would normally associate with Campbell as editor, though I've seen a few others like this from Astounding issues he edited.
Story summary.Thaddeus Funston, an inmate of a mental institution, is shaping modeling clay into something he calls an "atom bomb" - in a crafts class for inmates. That night, there is indeed a detonation of small atom bomb that actually did little damage besides destroying the crafts building.
Now the befuddled military guys investigating the detonation are in for some education...
Fact sheet.First published: Astounding Science Fiction, November 1959.
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Listed among the stories from John Campbell's Astounding/Analog.