Well, I'll typically start the season with a 22 or 20 ga. Thats just because of the leeves. Personally if I had to have one gun for squirrels it would be a 17. Last year I had many situations where I would have multiple squirrels on my tree area. And I would pick off the furthest one, and the others would run out to trees 40-50 yards away and I'd pick one more off. And then I'd usually get one more.
Last november I actually spotted several in the area that I was deer hunting and from about 120yds picked 4 off that were fighting over an acorn.

Yep they are ADDICTING TO HUNT!!!! lol ive killed um at about 80 yards with my 17 but i wouldnt be afraid to take shots at over 100 yards.