Here's what I use, Winchester 9422 with a scope for red/grey squirels, rabbits, etc. Haven't really shot anything at 100 yards, but 30-50 doesn't seem to be much of a problem. http://www.ssaa.org.au/OFFICIALREVIEWS/22leveraction/9422-WINCHESTER.jpg
If you cant shoot in Boulder county, then get one of these: http://www.mcbrosrifles.com/50_CAL_BMG.htm You can shoot from an adjacent county and still hit. They are highly accurate and used by some SWAT teams, plus they are cool. No worry about leaving the target suffering with this either.
I've got to support the Ruger 10/22. I've had many of them and still have one that is tricked out to look like a Mini-14. The gun is affordable, reliable, accurate and the .22 shells are extremely common--you can get a brick of 2000 for around $15. All you need is a nice 3 x 9 scope and those varmints will be history!
How 'bout a 375 H&H mag. That way you only need to be +/- 5 feet of target! Seriously, a 222 will do nicely. Also, a Benjamin or Sheraton pellet gun has served me well.
