The last possum that was unfortunate enough to cross my property during daylight hours got a nice pretty .22-sized hole through its ear and was very dead within a minute or less. I use my trusty .22 LR single-action revolver for small predators like that. Rifles are for longer range and the more experienced marksman, and shotguns are too messy and noisy. .22 plinker pistols are the way to go.