1mm = 0.03937 inches - so 8mm = 0.31496" (see above)
Why is the old British .303" Round (actually also an 8 mm) not called an 8mm? -I dunno (politics? - but the 7.62x39 Russian is the same diameter bullet .. actually 7.92mm or .312" ) - some European 8mm rounds use bullets sized up to .324". ** I've just read that in Canada the .303" round (still in use in No.4 Mk1* Enfields by volunteer units like The Rangers) is known as the 7.7x56mmR .
Yes - it is a mess..
Now when NATO were looking for a 'better 9mm' they got the FN 5.7x28 - a bottle nosed centre-fire .22". - why didn't they opt for an 8mm/32" calibre version of a short .300 Blackout cartridge (.308") aka 7.62x35mm and call it '8x28mm NATO' ?
300BLK Could be Shortened to say, 28mm to give an 8x28 MM ?
('7.62 or 7.92 x28' if you must).
I'm more comfortable with old English inches than centimetres and millimetres but why complicate things when there's a clearer, simpler way? - Many of what are commonly called '30 Cal' cartridges are properly closer to .32"/8mm.
My 'origami' 8x28 NATO (32 Calibre) 300BLK.
- a sort of stretched high pressure 32 NAA?
Bottleneck cartridges do feed very well into gun chambers - Wouldn't that work & impress better in a side-arm than a "centre-fire .22" ?