Going down in head diameter is generally always a problem.
The .204 was developed from the .222 Rem Magnum case.
If you want .20cal, you might be better off with a .20BR, which is a necked down 6BR cartridge which has it's origins from the .308 Win cartridge.
Quite a difference in cartridge OAL also if you're feeding from a magazine.
(Cartridge Dimensions courtesy of 6BR