The way I've had it explained to me, is that there is a secondary pressure wave which bounces of the base of the bullet. Upon ignition, the air space (including whatever the "filler" is) will compress and push the bullet forward, but the compressed air and stuff wants to go back toward the breech face - remember, compressed anything is basically stored energy - and this meets the still rapidly expanding propellant gas charge causing a localized radial pressure spike, which creates a ring in the soft steel of the black powder barrels.
The ring is usually seen about where the base of the bullet would be - it is very slight, and not a bulge seen on the outside surface. Do it enough, and it'll cause extraction issues.
There are some who will say I'm full of it, but I was in this business a long time, and I've seen enough rings I will not use nitro for black loads.
Besides, black powder is easier to get to shoot, and easier to clean the gun.
