.458 SOCOM
Hey, yeah, where are the 5.56 guys?
If there were still T-Rex's roaming around - there would be cajuns trapping it, climbing it, and shooting it at the base of the skull with a .22LR. They'd sell skin, meat, and bones, and there would be a Discovery channel reality show about them called "Saurus People".
10mm...
"Only way to be sure."