Try adding a tablespoon of citric acid but don’t leave brass in too long as acid will want to remove brass from the copper

 

Vinegar, acetic acid, leaches zinc from brass. Citric acid appears safe. Ultrasonic is fastest way to clean all soot from case and small units work well.
Hot water is all you need. Detergent, like Dawn, speeds up cleaning a little and keeps dirt suspended.
Citric acid gives a slight polish.
Of course, there is no improvement in ease of reloading or accuracy by removing all the soot, and case expansion can increase quite a bit in difficulty.
Thus, since all you gain is shiny, most go to stainless steel tumbling for ultimate shine.