An over-the-shot wad could interfere with the pattern, but how much and the significance of the interference would have to be determined. A wad under the shot charge is for taking up space. I think corn mill is sometimes used. Over the shot is usually to keep the shot from falling out of a rolled, crimped, or Hardin crimped hull. I have loaded 3/4 ounce loads in a 12 gauge but used no filler or overshot wad, so it depends specifically on the load data. I think the wads I used held a range of 3/4 to 7/8 ounce.
