I would say you'd be fine with either a 410 or 20 ga. The 20 ga will give you a bigger pattern and more range. I agree with the other, sit over those acorn plots and don't be afraid to sit for an hour or two. Last friday was the day before deer season opened and I sat down in a spot about like what your describing and after about 40 minutes two squirrels came chasin each other at about 40 yards. Shot the first one on the spot the second ran up the closest tree and after another half hour or so tried to make a slow get away and bam. The important thing is that you be patient.

1 more very important thing to remember: Have fun!! Any day sittin in the woods is way better than a day at work. Enjoy yourself and remember that you always learn from every experience.