My old Iver Johnson .410 and myself have taken plenty of squirrels. You do have to get closer to them obviously. For me it makes for a more interesting hunt. #6 shot to #4 is what I use.
A trick that is proficient in squirrel hunting: When a squirrel is on the base of a tree (from the ground up to where the tree starts to branch out) and sees you it will usually run slightly up the tree but more so go to the back, it go 180 degrees around to the other side. If you have a stick or rock near, throw is past the tree around to the back where the squirrel is. Almost all the time, it will come back around to your side so you can take a shot. This tip has helped me, especially when hunting with a .410 bore as you can get closer in between the time from when the squirrel goes to the back of the tree to when you throw your object and it comes back to your side.
Good luck, safe hunting.