I haven't done really extensive testing, but I've found that my Remington 581 likes Remington's Sub Sonic hollow points. When things are working right, it will shoot 1/2 inch 5-shot groups at 50 yards. They have more than ample power for eastern gray squirrels.
This ammo doesn't make as much noise as high velocity rounds. Last week I shot four squirrels from the same spot just by sitting still after each shot. There were 8-10 chasing each other through 4 different trees. They'd all hide at the sound of a shot, but after 5-10 minutes, they'd show themselves again. The only hard part was waiting till they presented a safe shot.
I'd like to think I can pick off a squirrel beyond 50 yards, but it's tough to get a shot that far in the woods where I've been hunting.