1 single .22 round to the head of a groundhog ( woodchuck ) will put them down. I killed many of them for local farmers when I was a kid and never lost one.I put three .22 rounds in a woodchuck about 2 months ago before it gave up the ghost. Not sure a pellet rifle would do the job in a humane way.
Funny thing about that woodchuck. I shot it the first time on a slight hill. It rolled into a ball and then rolled down that hill
about 20 feet. It then ran off a bit, stopped, so I shot it twice more. I didn’t know they really did the rollup thing.
I always used CCI mini mags 36 grain HP.