>>63961158If you put a bullet anywhere close to a T Rex's brain the bone fragments will kill it instantly.
Bell with a simple .275 against the thick skull of an elephant was able to penetrate the bone and kill the animal where it stood. He shot the thinnest part of the skull usually, but still, penetration depth with a caliber most hunters wouldn't consider was more than sufficient.
A more powerful single shot rifle, or an automatic rifle, loaded with appropriate ammunition, is going to rip straight through a T Rex skull and stop it before it reaches the hunter at any reasonable distance.