I use J & B Bore Cleaning compound. It's a mild abrasive which will remove lead without harming steel. I know that Brownell's has it, probably many other outlets do, too. Saturate a patch with the stuff, run it through the bore 5 - 6 times. Then, brush vigorously with a bronze brush that you've sprayed with solvent (or, as an alternative, spray the bore with solvent and brush with a dry bronze brush. Either works). Depending how heavy the leading is you may need to repeat the process. Finish off by running a series of patches through the bore. The first 4 - 5 should be saturated with solvent, after that, run dry patches until they come through clean.
I do this every time I clean, regardless whether I'm shooting jacketed or lead bullets. And, I clean after every shooting session. Over time the bore will get cleaner and cleaner and, eventually, will be pristine.
