If you own no other varmint rifles the place to start is with the 223 Remington. Everybody should have at least one. Abundant and accurate factory ammo plus it's a lot cheaper than many of the other calibers mentioned. 400 yards is well within the reach of a 223 Rem too.......The Savage Model 12 BVSS with a 9 twist has been my go to varmint rifle for 20 years now. Sure you'll want other guns in the future but it's tough to beat a 223 as the basis for your small caliber, varmint shooting stable.