Oddest obsolete calibre firearm I've seen was an early Spanish 1911 copy. It was a good few years ago and I wasn't sure of the legality of it. Never seen another obsolete calibre self-loading pistol.
there are some turn of the 20th century auto pistol caliber pieces that should qualify: stuff like the 9mm Steyr, the .30 Mauser, the .35 Remington. . .
UBC's Police Pistol Manager
"Nasty, noisy things, revolvers, Count. Better stick to air-guns." Sherlock Holmes, The Adventure of the Mazarin Stone