Granted government ties the hands of our military, and even LEO's at times, in what would be the best round for them to use. But the 5.56 is still a good round in my opinion. It would be better if our military were allowed to use soft or hollow point bullets. Even the Ruskies have adopted a smaller and lighter cartridge over the 7.62x39. Now, Kalishnakofh, spelling is probably wrong, was against any smaller or lighter bullet in his 7.62x39 AK designed weapon. But because of the proven track record of the 5.56, the Ruskies decieded to adopt the smaller, faster bullet. There may have been other concerns involved in this decision also. One does not really know.
The groundhog that you shot was apparently hit but if you did not find the carcass, you can't be sure exactly where you hit him. I have read stories of whitetail being shot right in the heart with 30-06 and still run for many yards before falling. A wild animal is a different animal than man. I used to do a lot of rabbit and squirrel hunting and I can tell you from my own experience, some are harder to kill than others. I have shot and killed every rabbit with one shot from either a 410 or 12 guage. I once had to shoot a squirrel three times with a 20 guage before I stopped him from crawling away. When I picked it up it had a couple of holes in the body as big as a nickle. When I hit it with the first shot it fell about 20ft. out of the tree and hit the ground running. And this was at a very close range. No more than 30ft. I shot him again and flipped him. He kept crawling and I shot him again. Of course he was wasted but I was not going to let him get away. This is a little gray squirrel not one of the bigger squirrels. They are just tough animals.
When I lived in Southwest Texas in the mid 80's a guy I knew shot a deer clean through the lungs with a 22 longrifle HP round. Went in one side and out the other. That deer ran for over 100 yards through a field before it fell. I shot one in the top of the head from a distance of about 75yrds with a Remington 22 longrifle HP and it flipped over and did not move. Dead. One shot, quick kill.

"If a man has nothing greater to believe in than himself, he is a very lonely man".