Look up the formula for "Eds Red"
Make up batch and after plugging the muzzle with a cork or rubber stopper, with the muzzle resting ''down'' inside a plastic container, fill the bore with Ed's Red and leave it sit for 24 hours.
The solution will penetrate under the lead/carbon fouling and is easily brushed out with a bronze brush.
As suggested, use powder coated or plated bullets if possible.
If you're casting your own, powder coating is a very simple and well worthwhile endeavor.