I've often thought that as the overshot card leaves the muzzle and begins to slow, it must have an effect on at least some of the shot. The recovered cards are always a bit beaten up. The cork wads are nearly impossible to find and if you do get lucky all you get are a few bits.
As I recall, we tried a load that went something like this: the powder and then a overpowder card with about a third of a fiber wad over that. Then we took a circle of thin chamois with some thin lube on it and pushed that into the muzzle about an inch and a half or so with some hanging free of the muzzle. Into this little "pouch"was poured the shot charge. The excess chamois was then folded over the shot and rammed home. Finally, the cork wad was seated on top of the whole affair. It took a while to get the size of the chamois right, but once we did it worked beautifully. Patterns were improved beyond expectations and were, to us at least, well worth the effort.
It seems likely that what was happening was that as the cork wad cleared the muzzle it simply broke up and had no effect on the shot, but more importantly, when the chamois cleared the muzzle it spread open and pretty much stopped and kept the wads behind it from blowing through the shot column. As the load travelled down the bore, the chamois may have kept the pellets on the outside of the charge from deforming since they had no contact with the bore. Naturally, leading was not a worry. With turkey season coming up, I think I'll give this a try again.
The gun used was a lovely old single shot late flint fowler in fine shape. It had one of those locks with the "waterproof" pans and was an elegant old thing. Don't recall the maker but he knew his job alright.
