Sitting the wads on their edge and pressing the back edge of a stainless kitchen knife into the edge will make a slight V allowing the air to escape. Some of the guys have to do this with waxy lubed wads for muzzleoading shotguns.

The wad still seals on the way out as it is pressed radially by the powder gasses and a wad above which obturates it out again. The groove needs only to be very small to allow the escape of air when pressing them down.

A lubed cloth 8 ounce or 10 ounce (.019" to .022") denim patch around a .600" ball would be almost as heavy as the .615"(true 20 bore) or .620" and it would eliminate all leading as well - just a thought. Weight would be just over or around 300gr. in WW and if Water or oven hardened, they'd probably penetrate like a 500gr. sold lead slug or better. I would expect both shoulders on a moose or elk if driven to around 1,800fps or above.