Squirrels are tough!
 

Popped this squirrel running along a fence post at around 25 yards with the ol' pellet/bb rifle. Dang thing fell between the wood and chain link fence. Shot it around 6-7 times with both bb's and pellets all going at around 600-700 fps. No effects showed until I hit it at nearly point-blank range just behind and above the shoulder blades with a pellet. Tough they are!

I have had some older squirrels (IE actual grey hair on the buggers from age) that I had to shoot more than once with a 20 gauge shotgun at close range. Last year I had one that I shot with a standard old 22lr round and he scurried off. I switched to CCI velocitors for all my rifle based squirrel hunting and have not had the problem again. For my shotgun I am now using Estate Cartridge #5s and have only had one squirrel since that gave me trouble.

GoodOlBoy