6.5 Grendel vs Button Buck

[VASCAR2]

From my experience head shots with cup and core bullets is very destructive to the bullet. There was obviously enough bullet mass to exit, evident by the exit wound. About any cup and core bullet is going to shed weight after hitting bone. The higher the velocity (closer distance) is going to generally reduce penetration. If your concerned about the 123 grain SST shedding weight you might consider one of the monolithic bullets (copper alloy). The partition bullets tend to hold more mass than a conventional cup and core bullet. A bonded core bullet like the Fusion or Gold dot are less likely to shed as much weight as the SST.

Obviously your bullet resulted in collecting the deer and I doubt the deer was able to go very far, if at all after that hit.