Wikipedia has a good article on the 9x21mm cartridge -
http://en.wikipedia.org/wiki/9%C3%9721mm
Frankly, I'm noplussed as to why so many Europeans are so 'down' on the 9x21. OAL stays the same as 9x19 and powder charges are identical. I would have no problem with buying/owning a 9x21.
But, if 9x19 is required, and you only have a 9x21 barrel, George Nonte has published his method of bushing chambers for shorter cartridges of the same caliber. I can attach a scan if anyone is interested.