For bunnies I put the bead in front of the nose, maybe even a little farther out if they are running broad side to me.
Kind of the same for squirrels, it's sort of hard to explain, you just need to get out there and do it and see what works.
If you aim center of mass with a shotgun at these little critters you will kill them, but you will be picking out lots of bbs.
It takes virtually nothing to kill a rabbit, they are not very hardy. One or two pellets in the head will do them in usually. Except western jack rabbits! They are like overgrown woodchucks with big ears

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I have actually killed a bunny with a .22 and didn't even hit it. I think I scared it to death. I fired and it dropped dead in its tracks. Never did find a bullet hole or any damage inside.
Paul