VERY thick fouling problem

I got a call from my friend the other day. He said his Mossberg 835 barrel had some kind of sludge in it that wouldn't come out. He dropped it off and I said I would get it out and bring it back to him.

This gunk is the worst fouling I have ever seen. It is in the middle of the barrel, covering the entire inside diameter over a 8" long section. It looks like someone went crazy with black silicone or caulk. It's thick, probably around 1/8-3/16 in some places. Very irregular shape. Over the course of an hour, 3 different cleaners, and lord knows how many brush strokes, it came clean. It came out in small chunks, none over 1/2" long. It had a rubbery texture, and was completely black.

I checked the bore that the gunk was covering up. No pits, no damage....just bright, smooth metal.

This build-up had accumulated in less than 30 rounds. It is not a new gun, but he was firing new shells through it. I believe he was shooting Hevi-metal non-toxic shot for duck hunting. I know it was raining opening morning, and he had water in his barrel from carrying it muzzle up.

So what is this stuff? If it was powder residue, the Hoppes should have taken it right out. I don't think plastic fouling gets that crazy, but I may be wrong. We are completely stumped.

We will be watching it closely to see if it happens again, and with what ammo. Thanks for the info guys.

I tried wrapping a 12 ga brush with a patch soaked in #9. Also tried CLP and gunscrubber. However, I did not let it sit for an extended period of time. This is all good information. I should be well armed next time I come across something like this.