We like to use a 204 Ruger for those pesky rodents but we have also used our deer rifles before it makes for good target practice in the off season. Might be a little overkill but it really helps for those times you get a 300-400 yard shot on them.
22.250rem 700 with the 26" heavy barrel. trigger set to 1.5 lbs shooting a hornady 40gr moly v-max. my best is 510 yards on a crawling woodchuck/ground hog/whistle pig. i would like to take a weekend and go to pa with my buddy to do nothing but shoot whistle pigs and help the farmer out.
Many, many years ago, a friend of mine invited me out to Montana. The ranchers out there had a big problem with the prairie dogs, the livestock stepping in the holes in the ground and breaking legs, stuff like that. So the ranchers didn't mind the occasional person coming along with a .22 thereabouts and whacking a few of the ornery critters. So we took a couple of Ruger .22 pistols out and drove along the edges of the ranchers properties taking pot shots at the little furry critters. Face it - .22 rounds are cheap, guns are fun to shoot, so what better way to spend the afternoon having fun and helping out the ranchers.
So I guess to answer your question - .22 long rifle in a semi-auto Ruger pistol.