Over the decades I've used the following rounds to shoot ground squirrels and prairie dogs: .17 Mach 2, .17 HMR, .17 Hornet, .17 Fireball, .17 Remington, .20 VarTarg, .20/.222, .204 Ruger, .22 Long Rifle, .22 Magnum, .22 Hornet, .22 K-Hornet, .221 Fireball, .222 Remington, .223 Remington, .223 AI, .22-250, .220 Swift, 6mm PPC, 6XC, .243 Winchester, .25-20 WCF, .250 Savage, .257 Roberts and 6.5-06. (There were probably some others, but those are what I remember offhand.) Oh, and have done some big-game practice on small rodents with rounds like the .375 H&H and .45-70.
These days I use a .17 HMR for shorter-range shooting. Used to use a .17 Mach 2 but grew weary of the ammo shortage--and then when some appeared, having to pay nearly as much as for .17 HMR.
For general shooting out to 300 yards or so I use a .17 Hornet with 20-grain plastic-tips at around 3650 fps. In the past have used cartridges from the .17 Fireball to .223 AI for the same purpose, but finally realized they weren't required--and also grew weary of trying to buy either .17 or .221 Fireball brass. .17 Hornet brass is easy to find, cheap, and the cartridge does an amazing job with only 12 grains of powder--which keeps the barrel from heating up nearly as much as cartridges using more powder. Plus recoil is so minimal I can easily watch bullet impact at any range, and a bunch of rounds will easily fit in a shirt pocket when I decide to hike around and shoot. (For ground squirrels often use the .22 or K-Hornet instead. They have the same virtue of minimal recoil and compact ammo, but aren't as good as the .17 Hornet beyond 200 yards. For ground squirrels 200 yards is plenty.)
For ranges over 300 have used everything from a fast-twist .223 with heavier bullets on up. Finally decided on a .204 Ruger for 300-500 yard shooting, since with 40-grain plastic-tips wind drift is less than with .224's of the same basic size. For beyond 500 yards I've come to the conclusion that a moderate 6mm (smaller than the .243) is about ideal. Am giving a 13-pound 6XC a trial this year.
The biggest virtues I found with the .223 AI were cheap brass, and no need to trim piles of cases. But the .204 has enough shoulder angle that when neck-sized it doesn't need trimming either. If I hadn't acquired a bunch of .204 brass cheaply during its first few years, would probably be using a .20 Practical instead.
