.204 ruger rocks, simply put. I have engaged horribly mean and nasty targets out to 600 meters (read, groudhogs). If i was in a position that i needed a varmit rifle, it would be chambered for this cartridge.
From what I understand it is a 222 necked down to a .20 cal. It is offered in 32 and 40gr. vmax bullets. Velocity is over 4000fps and supposed to be able to buck wind and be flatter shooting than the 220 swift and 22-250. Here is some info: