I have them on my bird feeder all winter. I shoot them ten pumps from a crossman powermaster 760. The feeder is about 10 feet from my sliding glass door so little velocity loss and not much accuracy needed. It kills about 1 in 5. I dont really care to kill them but all my efforts at trying to teach them to stay off my raised deck and bird feeder have led to little success. I used to use 3-5 pumps and aim for their rump, but I would hit them, they would run under the deck and come right back. Sometimes it took two hits to scare them off for the time being. Then, I began to realize that the same squirrels were probably coming back even after getting hit. So, I put out a live trap with Peanuts. At first, I would drive them a couple miles away and let them go. But that became a daily ritual. I dont know if they were coming back or if it was always a new squirrel. So, I decided to find out. I would catch them in the trap, then grab some yellow spray paint and turn them into a new species of yellow tailed squirrel. It was fun pointing the opening of the trap out and the opening the trap door. They would launch themselves twenty or so feet out from the deck and hit the snow leaving a yellow mark. But, you guessed it, I began catching yellow tailed squirrels. So I realized the trauma of being trapped and painted didn't do much to discourage them. I would even shake them up in the trap to traumatize them a bit more. I went back to the bb gun after a while. Besides, I was having to re-bait the trap several times a day with the tufted titmice stealing the peanuts. I even caught a great big Bluejay in the darn thing. It is the raccoon size trap. Towards the end of the winter, I was back to ten pumps and shooting for effect. Found a few of the darn things frozen stiff in the yard after the snow melted. Don't worry, I can go out in the back yard at any time of the day and be able to see at least one or two running around. There is no population problem around here.
Call me callous if you want but I tried. I really did. I even went as far as posting signs "squirrels not welcome - Violators will be shot" but they didn't head that either.