I used everything from .22LR to 6MM to .243 to 300 Win Mag (150 yard shot). 00 buck on the last one I got (48 yards with a Winchester 1200). Depends on how far away the dang whistle pig is at the time.
What follows is true: In 1973 we moved into a farm house that hadn't been occupied in a year and a half or so. Ground hogs were tame and numerous. Mom grew up around guns, but has terrible eyesight. A 3X9 Leupold on a 6MM Remington makes up for that. One day looking out the kitchen window she spots a GH at about 75 yards. Its a sliding window, no screen. Right about the time she rested the 6MM on the window sill, a Datsun 240Z (1st one I'd ever seen, you don't see many of those in NW Washington County) pulled in the driveway. BOOM!! Dead groundhog with tail spinning and all. Gravel flew everywhere, cause that poor lost guy (he was lost, no doubt) in the 240 got out of there in a hurry.
Ruger #1 A for 2014 is in .222
It'd be a nice walking varmint rig.
I've run .22lr, mag, Hornet, .222,.223,.22-250,.243 (lots) .357, .44, shotguns, muzzleloaders, compound and recurve, and even 250 grainers from a .35 Whelen (wooded setting with backstop).............I'm itching to try a .256 Win TC carbine
They are really bad around here also. I like to use my 223 but if they are bad enough there is no substitute for some 220# conibears. I usually run 10 traps at a time and can easily get 100 of them in a couple weeks.
I still use a 222 rem for ground hogs. Nice little round and because I have had it for years I have a very high confidence level with that rifle. 223 win would do the same. Where you have to reach out and touch em the 22-250 shines but I like them in heavy barrels and mostly don't carry it around as much.
I shoot about 10 to 20 a year depending where I am at on the farm. Started with a .22 but at the ranges I ended up being at, it didn't work out. So Then I upgraded to a 17HMR... That thar is a dandy round. I run it in a savage I got on sale at dicks... Great gun! This year I finally got my Frankenstein AR set up with a decent optic (Vortex crossfire 6x18...) Using American Eagle black tip.. nice big exit wound.. Cant wait for this spring to come...