Shoo,

OAL for both cartridges is the same so that they can use the same magazines. The 9x21 is loaded hotter than the 9x19, and the extra case length keeps you from using a 9x21 in a 9x19 chamber (much like .38/.357 and .45ACP/.460 Rowland). You might be able to use the shorter round in the longer chamber if the cartridge would headspace off the extractor, but it's not a good idea for reliability. It's also hard on the gun if the 9x19 does headspace on the chamber mouth, and if the firing pin is long enough to ignite it.

I wouldn't do it.

Buck