I had a 6x45 Obermeyer barreled AR15 at one time and I wished I'd got a wildcat based on the .222 case.  I was shooting that little Eagan bullet, about 70gr, around 2000fps and getting some pretty good groups.  I couldn't go much higher with accuracy, it had a 1/8 twist, and the borerider was a bit small. 5R rifling too.