Todd,
Like Ed said, it's got to be surgical. There is no majic hunting pellet. The best pellet is the one you can put where you need it to go. If not, don't pull the trigger on a critter until you can do that every time. Find the most accurate pellet for your gun, practice with it ALOT, and don't take shots beyound where you can't put all your shots in a dime sized target area.
I agree, 11 fpe at 13 yards is reasonable for human kills on Squirrels. But if you, the gun, and the pellet don't all work together to be able to hit in the brain area, your going to have runners.
The exception is, a head-on shot into the base of the neck, or into the nearest shoulder at the base of the neck if slightly quartering for a squirrel that's on the ground. Because your shooting slightly downward, those have a high chance of being a heart shot, while hitting other vital organs also.
Oh, and reload quickly 'cause some of those nutters are tough and don't always die easy no matter where you hit them.
Paul.