I haven't shot a Rock River, but I've heard good things about them. The AR15 would make a great varmint rifle, if it's put together out of good parts. Many of them will rival, if not outshoot a good bolt gun. Even the "not-so-accurate" ones are good enough for larger varmints, like coyotes, skunks, coons, ...whatever.

"Progress, far from consisting in change, depends on retentiveness..those who cannot remember the past are condemned to repeat it." Santayana