Ground hog shrugs off jsb 18.59 gr .22 headshot

Has the same thing happen to my Wildcat. I ended up getting a bigger fastener to replace the crappy stock FX one. No problems since. Something to keep an eye on for sure!

A bigger fastener please elaborate how to and where from

I got an allen plug that measures 16x1.0 from BelMetric.com - I replaced my hammer spring adjuster with this and dremeled the stock on my Wildcat so I can adjust it without taking the stock off. Not sure if this is the same for your rifle.
 
I once shot a gopher and was sure it was a good shot on it's head. It retreated into it's hole I watched the hole to see if it came back to refill and affirm or negate the kill when they don't. The hole went unfilled for about a week and then all of a sudden it was refilled which can happen when a new gopher moves in. So I setup and waited about 25 yards out and pretty soon it appeared and I whacked it. Turns out it was the same gopher that had a nearly healed pass through wound on both sides of it's head. Failed to hit anything vital. 

I once shot an elk with a 30.06 from 60 yards in the head seeing a huge blast of red mist through the scope so I knew I had connected even though the elk bolted out of sight. I sat still for 30 minutes and then began to follow the blood trail which led about 300 yards into some thick pines and there he stood leaning against a tree still alive. Head shots even with powder burners need to hit the right spot or they may fail to drop your target quickly if at all.