.223 Remington is the hands down choice. It is very accurate, easy to load for, mild to shoot, and provides all the performance needed to dispatch small critters way out younder.
For years I relied on the 52gr. Speer Flat Base Hollow Point in my .223 Contenders. Generally this bullet will achieve 2850 FPS muzzle velocity in Super 14 .223 Barrels, and is super explosive on small critters. I have also found it to be a very accurate bullet in several different .223 Contender Barrels.
However then I discovered the 40gr. Nosler Ballistic Tip. Now this bullet is super accurate in the Contenders I have tried them in, and it's ballistic coefficient is almost equal to the 52gr. Speer Flat Base Hollow Point. The interesting part about the 40gr. Nosler Ballistic Tip is, I have been able to achieve velocities up to 3350 FPS out of Super 14 Contender Barrels (safely) with this bullet while still providing excellent accuracy. Prairie Dogs don't stand a chance against this bullet out of a Contender.
If I would be allowed a second choice it would be either the 6mm-223 or the 6mm T/CU. Either of these is also super accurate, easy to load for, and provides excellent performance, actually a little better than the .223 Remington.
Another thing going for either of these cartridges is their mild appetite for powder, which allows more shooting before the barrel gets too hot to touch in an active Prairie Dog Town, and recoil is quite mild.
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