I've been shooting a M52-2 for awhile now. It also gets a build up of lead between the chamber and beginning of the rifling. I think what causes it is the abrupt transition from chamber to rifling.

There isn't a gradual 11 degree lead into the rifling like in a revolver, it's more of a square shoulder. I suspect if a gradual angle was cut into that area, the leading would stop. Been afraid to try that though because it shoots so well as is.

I've tried several brands of factory HBWC's, also cast my own. Have used several lubes, powder coating, and Hi-Tek. The ring of lead always comes back.

So basically, I'm no help.