If I get time, I need to post the section of the book I'm writing that deals with this. Short answer is that #5 high brass out of a full choke 20 gauge is the ultimate squirrel slayer. Long answer is going to take a chapter or maybe two.
I've shot squirrels with every gauge but a 10, every conventional rimfire round, a .32 caliber musket, air rifles, and various flavors of archery tackle. And a few with a blowgun and slingshot. At some point I want to get a hand loader and work up the perfect shotgun shell, but for now it's mighty hard to change from #5s when I want stew meat.
I've shot squirrels with every gauge but a 10, every conventional rimfire round, a .32 caliber musket, air rifles, and various flavors of archery tackle. And a few with a blowgun and slingshot. At some point I want to get a hand loader and work up the perfect shotgun shell, but for now it's mighty hard to change from #5s when I want stew meat.