Well, for those of you who have suggested I clean my guns more frequently, I clean my guns after each firing session. And use Hoppe's No. 9 with stainless steel bore brushes.
As to the fouling, it appears to be a combination of lead and carbon. As I said, I've no idea how many rounds have been fired through the gun, only that I've put less than 5,000 rounds through it.
And, again, all is well now.
Bob Wright