Actually my crud ring is always within 8" of the muzzle, never down near the breech.
I've observed that too. Not always but some days. Here's my theory on that:
The projectile leaves the bore, the gasses and lots of ejecta leave the bore behind it, the barrel is warm and the inside of the barrel tube is hot but it cools immediately, as it cools quickly, it sucks air into the barrel and the portion of the barrel in front of the crud ring (the first 8' or so of barrel) sucks all the moisture out of the air and leaves the fouling in that section of the barrel softened but the part of the barrel below that gets hit with dry incoming air (cause the fouling in the first 8' took the moisture out of the air) and the dry air causes the fouling to harden for a stretch of the barrel. At this point, the barrel has sucked in as much air as it's going to so the rest of the barrel doesn't get any effect at all. 'cept for the crud ring at the bottom that got made during the moment of inertial resistance of the projectile which caused a hot spot and also became a point of combustion for a major part of the powder charge.
whew...
