Lots of folks are using such guns for varmint hunting.Only down sides are weight and cost.I would go with .223.My CZ 527 is a .223 and it shoots great with plenty of punch for the largest coyotes.An important thing to consider here is twist rate.Faster twist's favor heavier bullets.I would suggest going here-
http://www.predatormastersforums.com...ubbthreads.php
Tons of info on all things related.
Good luck!