Actually it was perfectly round and just about .45 inch. The bone was visible in the x-ray and was an intact circle as well leading to speculation that it was caused by an implement used to slaughter livestock that actually punches a hole in the skull and into the brain.
The cockpit fire axe has a spike on one end and would have been something readily available.
From what I remember the hole was larger than a .45 and was about the size of the spike on the fire axe.