Odd barrel fouling ... anyone seen something like this?

[abomb60]

I was cleaning my MP5k-PDW today (5.5" barrel) and saw something odd about 1" in from the muzzle. There was a weird ring of carbon buildup forming a near perfect ring in there. I hit it with Hoppes #9 quite a few times as well as with a copper borebrush and it came out somewhat but I need to soak it or get something stronger in there. Cleaning it out doesn't bother me but i'm curious as to why it formed there in the first place.

Any thoughts?

 

[bpm990d]

Depending on the type of powder and the cleaning regime they can be tenacious. Most I've seen require some type of mechanical removal with a brush or a patch with some type of abrasive like JB bore paste.

I don't have any experience with that type of fire arm so I don't know why yours formed near the muzzle.

B