.22 Long Rifle in my opion is a good round for shooting Squirrels for eating. If you dont plan on eating em' and you are killing them to get rid of them a nice AR-15 .223 Semi-Automatic Rifle seems to work good. At least it works good for me. One Shot One Kill with that that Round VS a Squirrel!
.22LR to the head is best in my opinion. That is if you plan on having much left to eat. Besides .22 is a lot cheaper than .17 and it's not like you take squirrels from long shots anyway. Most of the time it's within 30 yards. Same goes for rabbits. Most important thing is find what ammo shoots best in your rifle. Mine happens to be CCI Velocitors and the Golden Bullets are a close second.
dependin on where you are if your in a moderatly populated area with 2 acrs of land a .177 cal is what i use, a .22 will carry up to a mile that wont be good cause a stray round may end up hurtin somthin or sombody but otherwise with enough land .22 is the best for that