No offense Ohihunter, but I respectfully, but totally disagree with the statement that the .17 HMR is a joke for anything but paper and squirrels.
If you are a serious long range groundhog shooter like probably most are, then a good centerfire is in order. I will venture a guess that many of the fine varmint calibers we have today were designed around the thoughts of groundhogs or rock chucks. However, when you add small game into the mix you are most likely talking rimfire.
On the other hand, a guy can be a serious groundhog hunter with a rimfire and have a barrel of fun. It will just be conducted at shorter distances. The rimfire is ideal for small game too, like rabbits and squirrels.
While I have mainly used centerfires, mostly .223, .22-250 and 6mm06 over the last 37 years, I (and friends) have hunted groundhogs with rimfires quite a bit (for the fun of it and to make a movie). We had a few shots past the 100 yard mark. We mainly used a .17 HMR and 20 gr. CCI Game Points, though we did use a .22 mag some and a .22 long rifle. We had one craw-off with a body shot from the HMR out of several taken. That is the only groundhog we lost and it was shot too far back, just a mishap in shot placement. The HMR is a fine groundhog cartridge in my opinion (considering rimfire). It is accurate and capable of head shots out to and past 100 yards. This statement is based on actual field experience, not just talk or conjecture. It is our go-to round when using rimfires. It will never equal a good centerfire and no one should expect it to. You just have to be a little more precise. We take head shots at every opportunity. If I were to give my hunting buddies a choice for groundhogs between the HMR, a .22 mag or 22 long rifle, they would choose the HMR hands down. So would I.