Your .17's have been slaying ' em nicely, from what we've seen. " Why ask why ?! "
I once killed a Soybeanus Digestus with a Hornady 55SX that skipped off the dirt, just in front of the animal.
When I walked up to see the carcas, I found that the core had separated from the jacket. And... as it turned out, just the expanded jacket had made it to the target, and was found lightly stuck in the groundhog's side...... by just the point of one single petal; looking much like a Japanese " throwing star ". The jacket pulled away from the skin w/ no effort, and the only evidence of impact was a tiny red spot where the jacket petal had made contact.
All I could surmise was that the jacket transfered enough energy to varmint to give it a valid kill.
Dead is dead.