That 'dirt' that you are seeing after flushing with hot water is flash surface rust and wipes out easily. Generally a couply dry patches then an oil or bore butter stops it from recurring.
I used to use hot soapy water with a hot water rinse and then coat the bore with bore butter. But lately I found a product at a stricly ML shop called turkey tracks black powder cleaner. All you do is plug the nipple, breech, whatever with a toothpick, dump about 2 tablespoons down the bore, cover the muzzle and tip the rifle back and forth to wet the entire bore, let set for 10 mins, pour out, remove nipple, breech plug and swab bore. The bore comes out slick as a whistle. I don't know what it is but it is not petroleum based. Ithen dry the bore good with some dry patches. Then run an alcohol patch down the bore to get out any residual moisture, then coat the bore with Montana's Bore Conditioner. Then I clean the breech plug or nipple and apply teflon tape (breech plug) and anti-sieze lube and reinstall. Wipe down the outside metal with gun oil and put away.