For PD's to 300yds, either one. You can kill prairie dogs with most anything, but there's killing and then there's gutpiling, then there's explosive launching. My shooting partner killed one at 230yd with a 17M2, but it just fell over. I've shot truckloads of them with 22Mag at 50 and 75yds and they gutpile ok, but at 240yds again they just fall over. Nothing to see. The 17Ah is great at close range but not real exciting at 250 or so. I've killed more truckloads with the 221FB out to about 200 and it worked fine, but I like bigger explosions. You can blow the crap out of them with larger cased rounds like 22-250, but the gun gets hot much quicker and you need to have at least a couple backup guns if there are lots of targets.
For what you're wanting the 223 case is a good choice. Enough powder for enough velocity to make them explode at closer range, and gutpile nicely out at 300. But not so much powder as to make the gun heat up or foul real quick. In 223 I shoot 50's and some 40's......mine are 14twist and even at 223 velocity the long Sierra 55Blitzking is very accurate. With a 6/223 I'd shoot 55's-60's.....the Sierra 60HP is explosive and very acccurate and should be cheaper than the plastic tipped bullets.
