Plastic shotgun wads are sized to fit into a shell, not the bore.
Most 12 ga. shotguns have a bore size of .720.
Wads are made with a cupped base that expands upon firing to seal the bore.
You may be able to use a shotgun wad if you get a cylinder choke tube.
That way, the tube will be the same diameter as the bore and you can force the wad in.
