17 HMR. but a 22 doesn't scare them as much, and may give you follow up shots or multiple shots at different squirrels. If you are not shooting and distance I would sat the 22 is your best bet. 22 mag is also a good choice.
Shotgun would be my bet if you think there may also be grouse in the area as well, and can take them while walking from spot to spot.
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