I use a Mod 70 Heavy Varmint in .223 with handloaded 55 grain VMaxes loaded to 2985 fps average for "close range" pdogging, which I consider to be 325 yds and less. I use a 6mm Rem with handloaded 75 grain VMaxes out of a Rem 700VLS for the long range stuff.
I've gotta agree with nmleon re: the .223 and truly long range combined with the wind you constantly face in pdog country. I've popped a few long dogs with my .223, the longest being 394 yards a couple weeks ago. That said, that kind of range is better left to the .243/6mm class of rounds if you want to consistently connect.
First off, I'll be the first to admit that a .223 can be sighted so it is consistently effective at 400 yards, but, you'll find yourself missing a high percentage of "no-brainer" shots in the 250-300 yard range. That said, the folks I've seen that "consistently" kill pdogs at 400+ with the .223 are usually AR shooters, and "walk" their shots onto a dog. Personally, firing 500-600 shots to kill 100 dogs is a little expensive for my taste, even though I reload.
If you want a good 300 yard or so pdog gun, it's pretty darned hard to beat the .223, but, if your bag is shooting consistently longer, without sacrificing close-in accuracy, the .243 would be a better choice.
JMHO
Mike