Both of the above pretty well sum up the science of chamber ringing.
One thing Ive always wondered about is just what types of bullets were utilized when ringing occurred.
My bet is this.
Since most nitro for black loads are used with full groove diameter bullets, I would think that the bullet, especially one of jacketed construction, would act as a momentary obstruction as it meets the rifling causing the pressure spike described above which is the obvious culprit in chamber ringing.
Has ringing been observed when using comparatively soft all lead projectiles?
Havent heard anything definitive about that.