Since reading this, I can't seem to get the image out of my head of what a .300 Win Mag would do to a prairie dog!
Just think of a sudden puff of Red Mist that quickly falls to the ground.
Actually, a lot of overkill Cartridges will do that. Had to shoot a Squirrel suspected of being Rabid with one of my 7x57mm Mausers once, as I did not have a .22LR or .410 with me. Not much left to send in to the test lab and we built a Camp Fire on the surrounding ground area.
Getting back to the topic, you cannot buy one gun to do all things well (aka Jack of all trades, Master of none) but you can buy a gun that will do many things very well, and if I did not have the Battery of Rifles I currently have in my Vault, I would be shopping for a .30/06 Bolt Action, preferrably a Mauser of some kind.
A .243 will go nicely in your start of a Battery of Rifles, but you'll need a .30/06 down the road.