9mm IS .357 inches; ergo less than “4”.
10mm is .395 inches and is a more powerful cartridge. It may qualify as an honorary .40. But then, how many obscure calibers do we need?
In my opinion, the only reason for carrying a .357 sig (other than an affinity for obscure calibers) is that you are Asian and regard “4” as an unlucky number.
On that note, I have a friend (Asian) who owns two Sig’s - a 9mm and a .357 sig. You may find it difficult to believe but the brilliant guys at Sig Sauer designed the P226 so that clips from the 9mm will fit in a P226 chambered for .357 sig. My friend accidentally slapped a 9mm clip into his .357 sig at the range and fired off a round. It discharged but the badly expanded case stovepiped. Didn’t seem to hurt the gun though. The moral I guess is, if you must own two Sig 226s, always leave one locked up at home along with its ammunition.
And if you’re Asian and don’t want a 40 something caliber weapon, plunk down the big bucks and get a Desert Eagle.
I’m aware of what caliber a 9mm measures; my point was that the original phrasing could’ve been a cute way of drawing a line between the inadequate .380 and the 9/40/45, since 9>4.
And despite what 10 millimeters converts to, if a 10mm cartridge fired a .395 caliber bullet then there are a lot of people shooting .395 S&W right now, since the .40 is just a chopped down 10mm case that fires the same bullet.
(Albeit much, much faster...energy on par with the .357 Magnum, or 50% more powerful. I still don’t understand why the FBI declared it too powerful for everyday use...everyday use is holstered. On the rare case when you have to fire, why wouldn’t you want a substantially more powerful but still very controllable round?)