I was just wondering what is the difference between the new Ruger .204 and the existing wildcat Tactical 20, a .223 necked down to .20cal?

The .204 Ruger uses the .222 Rem Magnum case necked down to .20 instead of the .223 case. They are very similar but not quite the same. However performance is apparently nearly identical according to reports for .20 Tactical users. Plateau Hunter

The 222 Mag has more internal capacity than the 223 and should be able to more reasonably reach the holy grail of 4,000 fps.

With respect to the .204 flopping everyone has been predicting the same thing for the 450 Marlin but life goes on....I suspect they have a real winner as the only reason I haven't had a 20-caliber built was the pain-in-the-butt reloading is with those teeny bullets...it would be nice to be able to buy off-the-shelf if you wanted to..if Black Hills Ammo starts production you know it's a winner.