As someone reminded me this weekend... I recall a callow youth who discovered that surplus military 7x57 ammo was perfect for "barking" squirrels - by shooting through the tree when the squirrel was hiding on the backside. Usually no visible wound... I would *never* do that (now), of course... besides, there's a subdivision where those trees used to be. And squirrel season is finished for the year. And whoever that kid was, I'm pretty sure he no longer has that rifle and 2 cases of surplus ammo...
However, I have taken a fair amount of headshot grouse with a Ruger Blackhawk 45 Colt (heavy loads), which is actually easier than it may sound, two headshot squirrels years apart with the same (on a bet, and yes, I ate them), and some small game for the pot with my 30-06. Ok; the 30-06 was with reduced loads and a hardcast gascheck target bullet...