I wasn't there to witness it, but my dad shot a groundhog at about 175 yds with his Marlin .22. He will be the first to admit it was a good bit of luck. He just aimed a bit high and shot, and killed it. It wasn't until a few months later that I went and ranged it (it was from our cabin to a yearly hole in the lower field, so I know it was the right distance). So that is the farthest.

I've also seen him take a crow out our back door at about 150 yds. Those are the exceptions though, most shots are under 100 by a great deal.