I went shooting yesterday, a rare treat for me these days, and fired 4 black powder cartridge handguns and a rifle. Had close to 100 rounds of dirty brass when I was done. Didn't get to them until about 1830 hrs or so last evening and was done with the guns by 2000 hrs. Had I taken time to boil water for each gun, and then tried not to burn myself on hot metal during the boiling water treatment, it would have taken at least another hour.
I just used tap water with a dash of Simple Green (too make the water wetter) and scrub with a soft bristle round brush or a tooth brush for the corners. I wipe everything dry with an absorbent cotton rag, then run over everything with a patch of Ballistol. Haven't had a gun rust or corrode in all the year since I came back to BP cartridge. YMMV
Dave
PS: The dirty brass gets scrubbed with soapy water then dropped in a bucket of clean water. When done I dump the brass into a colander to drain off the water. A couple of shakes to get rid of the excess then I spread the brass out to dry on an old towel. When it's dry, into the vibratory tumbler for how ever many hours it takes.