There are three distinct types of wads. The Over Powder Wad is usually a hard cardboard wad that is 1/8" thick. The Cushion wad is made of softer material, often celotex, the insulating wall board use in housing construction, that is 1/2" thick. Cushion wads are usually lubricate before being used, and there are pre-lubed cushion wads sold that are made of materials other than celotex. You will also see home made cushion wads being used by shotgun shooters, punched out of synthetic materials, like those nylon sponges, or styrofoam insulation sheet. The thickness can vary from very thin, as in the styrofoam trays used under packaged meat, to 1/2" thick pieces cut from sheets of styrofoam sold by home supply stores for home insulation.
The third wad is called a " Card wad" or " Over Shot Card ", because it is made from 1/10th" thick card stock, the same stock used for shirt backs when you buy a new shirt at the store. The thin cardboard has a hard, shiny surface on both sides. It is used to hole the shot in the barrel until the round is fired.
Traditionally, shotguns were loaded with a powder charge, then an OP wad, then a lubed cushion wad, then the shot, then the OS card on top. Early brass casing shotgun shells were often loaded with almost the same components. Even the early paper, and plastic shotgun shell were loaded with similar components, but smokeless powder was substituted, and often different or thicker cushion wads were used to fill up the difference in the casings. The OS cards remained in use as long as cartridges were closed with a roll crimp. When the modern shotgun shell, made of plastic, and using a plastic wad and shotcup to hold the shot, and seal the bore was employed, the mouths of the cases were crimped with a 6 or 8 point star crimp, rather than using the older roll crimp. Other than design work on the shotcups and the wad bases to the cups, there has not been much development in shotgun shells in the last 50 years. New powders are being used and offered, but the plastic shotcup/wad combination insert is where the only real competition has occurred between the manufacturers.
If you own a cartridge gun, and want to shoot BP in it, I recommend buying the Brass casings for this purpose. You can shoot BP in plastic casings, but it burns the plastic and melts what it doesn't scorch, weakening the casing to the point that more than maybe 2 reloads is pushing your luck, and endangering your gun. The brass casings can last 3 lifetimes, if care is taken in how they are handled and loaded.
For ML shotguns, stick with the tradional wads. Jim Rackham has suggested that using only OS cards makes hunting a much simpler process, in that you don't have to carry around some of all three wads. He uses 4 OS cards to replace the OP and cushion wads, then uses 2 OS cards to hold his shot load in the barrel. Poke a hole off-center in each of the OS cards to help you seat them easily down the barrel, and just turn the cards so none of the holes align with the hole in he card next to it. When the cards leave the barrel, air separates the cards, where they quickly fall to the groud, and don't follow the shot, and " BUMP" the shot to create a " donut hole pattern", as seems to happen when lubricated cushion wads are used.
