what you found was the crud ring.
Certain primers are HOT and when shooting pyrodex, american pioneer, T7, it leaves a crud ring, Its a hard crusty carbon build up. In order to stay clear of this, you MUST swab the bore in between shots to remove powder fouling.
You MUST mark your ramrod - range rod so you can if the bullet is off the powder charge or not. Leaving a gap between the powder-bullet can cause either a bulged barrel or worse, a burst barrel.