Ugh making me want to shoot squirrels! I have only ever eaten fox so ill have to try n find some greys. Lots of good advice here. Only thing I would add is that they make a lot of good squirrel calls that I have had some luck with. I use a wounded squirrel and a standard bark call. Wounded will get them out looking bark will get them to sit still.
12g with 6-7 shot may sound like overkill but they are tough animals. I started out as a child using a 410 and my dad would carry a 22 for the longer shots. 20 and 12 would work but I like having a 22 along.