525 fps @.177 cal should do the trick.
But sometimes you gotta hit em twice.
Try and get a good shot off when they are on the ground,
Put out some peanuts or bird food on the ground , and nail em while they are pigging out.
Ya gotta shoot em ALL.
If they have had babies in that location, the little Bast###ds, will come back.
they think it's there home and will return, I know I had the same problem.
They will chew wires in your walls and celings in the attic area if they can get to them.
They have caused house fires, by stripping wires bare, causing them to short out.
If something like this Should happen, DON'T TELL the insurance company it was Squirrels.
They DON'T cover RODENT Damage.
Tell them you think it was a Racoon, that most policys cover.