That would be the infamous Triple 7 crud ring. I haven't shot that powder so I can't be much help to you as for how to minimize it, but there are some here that don't have the problem as bad. I know for one I would probably try loose powder over pellets. I'm sure those with more knowledge about it will chime in.

Vince
P.S. Welcome aboard. Be careful how much time you spend here. There's been a weird bug floating around the last couple months that appears to be highly contagious and costly to treat. If you insist on being here, might as well start making room in the gun safe.