The crud ring is magical and mysterious. It comes from Triple7 but other powders have been known to produce a similar, though not as hard, deposit in the bore's area of ignition. The powder can be loose or pellet, FFg or FFFg, in large or small amounts, and it can be Monday or Saturday. Your buddy's exact twin of your rifle may get it while yours doesn't. Once you get it, it can't be cured and you will likely infect your other rifles with it. There are many prescriptions around the web for minimizing the magnitute of the crud. Those include cleaning methods, methods for drying your bore before the first load, preservatives, using milder ignitors, polishing that area of your bore, and some minor voodoo rituals. None are fool-proof. Throw some sugar in a very hot pan and you will see something deposit on the pan similar to the crud ring, then you'll wonder why the heck you did that.
Did you ever get a cut on your hand without knowing it, only to look at your hand later - see all the blood flowing and suddenly realize something hurts like mad? I haven't.