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.
![](http://www.69hdflh.com/untitled_clip_image001_0002.jpg)
Quite a difference in cartridge OAL also if you're feeding from a magazine.
(Cartridge Dimensions courtesy of 6BR
![](http://accurateshooter.net/pix/204Ruger.png)
![](http://accurateshooter.net/pix/308Win.png)