I was allways told that a shot at the back of the head is a familiar style of Soviet execution. Germans shot their prisinors aiming at their forehead.
Well, I've never personally seen where fascists (or soviets) shot at. But I was told (many times, by many people, some of whom, probably, have seen that) that all prisoners of war, who were for some reason executed, were shot at forehead, while sitting on their knees , or tied to a stick (i.e. a tree), eyes tied, and shot at the front by a row of soldiers (when there was a time for such rituals).
But mainly POWs, and all other victims, were killed in any way it suited their slayers. So I'm not sure that a hole in a certain side of the scull may be presented as any kind of evidence. After all, it's not DNA or smth. Any willing side could scene it (the execution) in any desired way.
However, this quite long BBC (if my memory doesn't fail me, and I think it doesn't) movie is not about "who killed those polish officers in Khatyn'" (sorry if the spelling is wrong), with all due respect to the memory of the victims. You (I mean all of you guys, not just Paciat) should probably take some time and watch it through. It's really decent in ways of historical accuracy and quite informative for almost anyone, who hasn't studied the history of WW2 in precise detail.