Keep in mind, no matter how many you kill, if you have a bird feeder you will ALWAYS have squirels. So if SQs are a problem you need to rethink how important that bird feeder is to you.
Learned this trick from a pharmacist. Crush 2 to 4 percocets(sp)(leftover from surgery) mix with peanut butter set in the middle of the yard, ( make sure no pets are around) and wait. Had a squirrel eat some up climbed up a tele-pole and splat from 50 ft. Died with a smile on his mug.
John