I remember when I was a youngster on Sunday’s I would sit in the barn with a 12 gauge double barrel shotgun and protect the garden from groundhogs. The most I ever got at one time was three, which is pretty amazing with two shots. In reality two came up side by side and I only winged one so it took a third shot to complete the task. The rule was if you found the hole you could put the carcass back into the hole, but it you could not you had to bury it. It was a lousy afternoon when you are digging a hole to plant a dead groundhog. The very worst thing about a groundhog is how they smell. If you can, envision a rodent which never takes a bath and spends time resting underground you begin to see the tip of the iceberg.
I had a Great Uncle who was a farmer and he would sit in the bed of his truck in a chair and had a couple of small beanbags he would put on top of the cab. I don’t recall the number of groundhogs he shot in one year, but I know it was over one hundred. His firearm of choice was a rifle and I believe it was a 30-06. My father recently sent a note he had killed his fourth groundhog this year and it is his fourth groundhog he has bagged by shooting from an upstairs window. The latest was killed using a 12 gauge with a slug barrel using shotshells which is not an ideal firearm combination for groundhogs since the barrel length and lack of choke will allow the shot to spread wide in a shorter distance. Earlier this year he killed on using a .380 ACP pistol.
If you do decide to go groundhog hunting they always appeared to me to have a keen sense of smell and hearing. You have to wait very still until they are close enough for the effective range of the rifle you are using. If you are using a shotgun, I always preferred #4 shot for groundhogs and generally was shooting at 30 yards or less. If a rifle I think a well placed .22 would do the job, but it might be more humane to use a larger caliber unless you can be sure you will hit a vital area.
I once saw a groundhog up a tree, so don’t forget they may head for higher ground if startled.