Rich P,
I think nicked around the edges is better for wads than a hole in the middle, (sorry Mike!) because the ramrod often covers the middle hole.
Someone asked if we thought this sort of destruction Daryl showed, would be more common with percussion than flint barrels, but I reckon not, as the gasses are expanding so fast, the touchhole could not cope with this sudden pressure, nor act as a safety valve.
With shot, I too have often found the rammer being forced back up the bore by air pressure when seating the over -shot wad. I just press it down again and hold it down, till the air has escaped. (Or chew the top wad 'round the edges to make air escape routs)