Sorry to say,
But all the Excam revolvers I have examined have been inexpensive, poorly made guns. They routinely have casual alignment of cylinders and barrels, oversized and badly cut forcing cones, and timing issues. Many NEW guns have these problems out of the box! All of these things cause or contribute to lead spattering and fouling at the barrel/cylinder gap.
The only suggestion I have to do as the others suggested: try copper plated bullets and see if it makes a difference.