What happens with a wad on the powder is that symmetrically confined powder will form a uniform front, which, when it impinges on a stationary object, forms a high pressure ring.   This wave reflects back down the bbl, with some of the high pressure gas passing on the outside of the now-relaxed cartridge case.    WHen the pressure wave progresses to the rear, the gas that has gotten on the outside of the case collapses the case.  A known way to prevent this is to slightly bell the neck of the case for each shot, so it is in contact with the chamber.
     When the powder is not symmetrically confined, this phenomenon will not occur.  This can be achieved by not putting the wad clear down against the powder.    Reading the experiments of Dell and Helbert will make this clear.    Charlie, e.g., found that the effect could be produced WITHOUT any wad at all, simply by firing the rifle straight up.  Charlies' practice was to seat the wad close to, but not touching the powder, thus promoting uniform ignition, and also placing a cork wad in the mouth of the case to prevent damage from powder grains impinging on the base of the bullet.  Whether the powder grains peening the base of the bullet is a problem or not is a whole 'nother controversy.

CHRIS
RGChristensen