Although I don't know of any Ruger .204 in semi-auto(they make a conversion kit for the AR-15), it is a great caliber. Not overly loud, shoots a small, VERY high speed bullet(4,000 fps plus) has great ballistics as well. I tear ground hogs up with it at 400 plus yards. It'll take care of any beaver, coyote or musk rat out there... It's a newer caliber that I have come to love...

I do have a Ruger 10-22 which is cheap to shoot, crazy reliable and a ton of fun as well. Mine is all tricked out with after factory everything. .22 lr is a bit light for beaver as others have said... If you can find one, the Ruger 10/22 in the .22 WMR may meet your needs. http://hunting.about.com/od/guns/l/aast1022magnum.htm

You can never have enough guns!