"Studies suggest a T-rex had enough muscle in its jaws to build the same crushing power in its bite, as an entire elephant sitting on something."
Yes, and once again, that translates into construction more than absolute durability via thickness.
This is a 1/4 scale cast, but it shows what I'm talking about.
The skull is VERY strong and heavily constructed where it needs to be strong to support the musculature associated with its bite strength -- the lower rear jaws, what we could consider to be cheek bones in humans, and the rear of the skull.
But notice over the top of the skull, over what we call the nose, sweeping back towards the eyes and ears, and over the frontal area that on humans would be the forehead.
The bone there isn't nearly as thick, and there are very large areas where there's no bone. But what there is is construction. That part of the T Rex's skull looks almost like the arched windows in a Gothic cathedral. Evolution designed the T Rex's skull so that the bone was thick where it needed to be thick (muscle attachment points), but not nearly as thick but VERY structurally strong, in areas where bone mass isn't critical but structral strenght is (in this case, to provide the platform for the T Rex's bite).
There's absolutely no magic to that, and it wouldn't be all that hard to get a bullet into the T Rex's brain because there's not much protecting it. It's all structural engineering.
You see reverse construction on other dinosaurs where the heads were used differently, such as the bony skulled dinosaurs like Pachycephalosaurus. It's thought that such dinosaurs had extremely thick skull crowns and very strong neck muscles because they either used them for defense or for fighting for mates (think big horn rams). Those dinosaurs would have been problematic to get a bullet into.
