The shot you describe to the head would flatten that squirrel easily with 750fps.
I've done that countless times with a .20 sheridan rifle that has less power. I can't imagine what is the issue if that squirrel was hit in the head with a pellet from that rifle and did not fold stone dead?
Maybe head shot is not brain shot? The head is much bigger then the brain. Body shots are different, they can run to a nest and die inside. But an actual brain shot is game over.
There is likely truth to this, especially the part of precision and anatomy, so I mounted a higher power scope to make sure I can see more detail next time. It was hard to tell eye from ear with the RWS 4x scope that came with the rifle.
