I run Highland Games at some blackpowder Rendezvous in my area. I'm familiar with the vague standards for the Caber (although we try to keep ours on the smaller side of the scale, we're only out there for fun). I've always been skeptical of the idea that the event originated from the practice of throwing a log to bridge a stream in battle. It seems that you would need more reliable accuracy at a time like that, whether you were attacking or retreating. I suspect that it may have had it's beginnings during logging or land clearing, when our competitive natures led us to make a contest of part of the task.