Must be pretty hard to measure, with the rifling so short AND progressive. I've met only one positive figure on this topic - the Buturlin-Matska Paradox, 20 bore 24 gauge, had 1 in 900 mm. And 7 grooves, just like Fosberry's patent. The Russian sources have it as if Buturlin invented small-bore Paradox rifleing, a few years before Holland and Westley came up with their own 20 ga. paradoxes (the hint being that the wily Brits picked up the idea from an innocent naive Russian lawyer-ornitologist-gun guru). I'm wondering.