I suppose it could be a matter of whether you measure the bore diameter or the groove diameter. For instance:
"Hastings Paradox slug barrels - The bore diameter of the 12-gauge Paradox rifled barrel is .716 inches and its groove diameter, .727 inches."
http://world.guns.ru/shotgun/sh30-e.htm lists it as:
KS-23 rifled bore of 23mm caliber
Type: pump-action
Gauge: 4 / 23 mm (rifled barrel)
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I've read of 4 bore shotguns measuring .935" to .955", so multiplying .935 times 25.4 mm = 23.749 mm
Other sites list the KS-23 as 23.2 mm, so it's close.
Heck, I dunno, I'm just winging it.
John