Well a crud ring is an accumulation of burnt powder and residue. Normally it is found in front of the breech plug where the actual powder ignition under the projectile takes place. Depending on the powder you shoot the crud ring can be very hard in nature, to very soft or even non existent. It is also a residue of sorts or fowling. But the crud ring people worry about can cause loading problems if left unattended. To remove it, you simply swab the bore with a slightly damp patch. It will remove the fowling and crud ring from the barrel. This helps the loading of the next round.