Usually leading looks like "chunky" deposits at the rifling at the rear of the barrel, and as build up on the forcing cone.

In the chambers it looks like a sort of crusty enlarged ring where the chambers taper down near the chamber mouths.
A clean chamber will show a smooth ring-like area where the chambering reamer cut the chambers.

The Scotchbrite I warned against are the green abrasive flat pads sold in grocery stores and in auto and hardware stores in other colors (grits).
These should never be used in a bore or chamber.

What you were looking for is 100% pure copper Chore Boy scrubbing pads.