I'd also suggest something smaller than the 22-250. It's loud and will potentially mess up your trap... not to mention, unless you're shooting from several yards away, you're going to get hit with dirt and raccoon splatter. (possibly metal fragments too)
Did you use the peanut butter or strawberry syrup? Opposums usually come in to clean up what somebody else has killed. Keep your eyes open. Good choice with the 22.
Ya...Possums usually come in for easy pickings after a kill by a racoon or other predator. It probably did not do the original kill. I would keep the trap set and keep watch for a few more nights. Usually a racoon will take a day or two "off" and then return.
Sorry for poor Sam.
Can't believe I am awake at this hour and on this site. It truly is "addicting".