OK - It's pretty obvious that the "except for the area between the breach plug and about 5 inches up" is the problem. You'll have to work on that to get it clean. Follow cayugad's advice and I'm sure it will solve that.
Since you're using Powerbelts, the sabot issue doesn't apply. They ARE some of the easiest loading projectiles out there, so what you'll have to do is figure out why that 5" isn't clean (and/or smooth), and how to keep it that way.

Good Luck