Best long range prairie dog gun?

If you have an AR platform you have a great chance to perform as well as bolt actions these days. You can get a heavier barrel with a 1 in 7 twist,get some optics to reach out farther,something like a Leupold VX-3 6.5 x 20. That way you can use 70 or 72 grain bullets to buck the wind a little better,and you can carry energy down range that little bit more. Added a link so you can see what I'm talking about for a new upper,if you even need that.

If you decide to go with a bolt rifle,almost everyone have a varmint variant;Savage makes a single shot Left Feed,Right Eject model (don't remember off hand, Model 12 maybe) and of course Rem,Ruger,Browning,Tikka,etc all make a heavier barreled version of their standard rifles.

600+ yards is amazingly pushing the barriers of the .223 cartridge, in spite of heavy boat tails. .243 Win. might want to be my first selection for groundhogs at that selection. The 22-250 burns barrels up, and the 7mm-08 might want to nicely be overkill for groundhogs. Clay pigeons make superb practice aims. when they damage into large products, next, attempt capturing the man products. Makes exciting, large practice. appears like a superb platform too.

* Your Best Choice is the 22-250.* I have the Remington Model 700 Varminter in 22-250 with a Redfield Model 3200 in a 20 power scope on it.*

There is no "best" firearm for anything, between opinions and different circumstances. If I were to start from scratch again though, I'd probably get a .204. Very flat trajectory, long range, and it doesn't go through barrels like my 22-250 does.

Rem 338 mag

A quality scoped rifle in .22 magnum, or .17 cal HMR would be ideal for this purpose.High powered rounds in .243, .270, and .308 is overkill and a waste of money, which could be use for other things.

220 swift

The .204 Ruger is the caliber for you.

308 bolt scoped to the max.

.25-06.