It seems unlikely[...]
That's not a particularly substantive comment. Got anything to back up what seems to be the case, according to your opinion?
the energy effects of a 6.5 mm round at range
Compared to:
those of a 5.56 mm round point blank
The behavior of either bullet on a human head, given the circumstances, won't be radically different. Hydrostatic shock, inducing spin on the slug, upon entering the cranium, is going to pop the skull open on exit, and everything in between is going to be shredded hamburger.
The head wouldn't be ripped clean off. It wasn't a 12 gauge shotgun inside the mouth. These weren't explosive shells. They were ball rounds.