You might say it is analogous to bulging a barrel by having one bullet stuck down it when another is fired. In this case the second bullet is like the "shockwave". In chamber ringing the shockwave is caused by the level surface of the powder perpendicular to the length of the case propagating (and intensifying) down the case. Having the bullet seated in the rifling is not a problem. This is a standard Scheutzen practice. The bullet is seated about 1/16 inch ahead of the case and often there is a wad at the mouth of the case to prevent the powder from spilling into the chamber on loading. No problems there because the powder is more or less angled along the length of the case and the burning is happening in more random directions.
