I went squirrel hunting with my new .17 HMR this Saturday. I thought it would be a sweet little rifle for small game, but I was wrong. If you do not hit the head, it is a squirrel wrecking ball. Really fun to shoot and the ammunition is pretty cheap, but way to much gun for tree rat.
DairyFarmer is right..the .17 HMR may be smaller but you'll hit WAY harder than a .22 LR because of the increase in muzzle velocity and the kinetic energy is proportional to the velocity squared.
See
http://en.wikipedia.org/wiki/.17_HMR#Comparison_with_.22_LR_Ammunition esp. "overly destructive to small game at close range" lol
