Max effective range on the .223 is 460 meters or a little more than 500 yds. The drop of a std military round is about 18" at 300 meters. A whistle pig (praierie dog) is pretty small so there will be more of a challenge at closer ranges...say out to 250 meters. Most any variable scope will do the job. You don't need a sniper scope. You don't even need to spend the $$ on a top of the line model. I'd go with a Burris or TASCO or Bushnell 3x9. I'd recommend a fine cross hair reticle so as not to obscure the critter itself with the posts. Maybe a 4-plex reticle.