My recommendation is.....DO NOT USE A 22 ON A POSSUM !!!
I speak from experience. Trust me here. It is cruel and horrifying to watch. It can take 10 shots or more to kill one even with potent rounds such as CCI Stingers.
The only way to drop one in one shot it to hit it square in the brain and even that usually takes 2 or 3 shots. They make the most horrible screm when they are suffering and it will really bother you if you have any compassion at all for animals.
Rodents like possum and armadillos are tougher than you might think. There's no need to be cruel on that level.
Use a 223 and they die in the blink of an eye. If you can't use enough gun then don't shoot.
Also, a hollowpoint won't help. They won't expand much in such a small target. I've even tried Quickshock fragmenting 22 and it didn't work either.
There are lots of options for you. You can buy stuff to sprinkle on your yard that will keep them away. Don't leave tasty garbage where they can get to it.
Believe me when I tell you that killing a few here and there will make no differance in the population. You won't be helping as much as you think by shooting them.
They won't really hurt you if you don't confront them. We have them here and it's not a big deal although they do scare the crap out of my wife sometimes at night.