I usually tumble with stainless pin sand some Dawn dish washing detergent.
Some cases come out with some minor black stains, which I usually just inspect for pitting and then reload.
If any cases are still completely dark, I discard those because I suspect that the oxidation has progessed too deeply into the brass to trust at high pressures.
If I want to get the minor stains off, after tumbling, I use an ultrasonic cleaner with a 1:3 dillution of white vinegar in water, which usually requires only 5 to10 minutes.
Whenever I use an acid to clean brass, I after-wash in a solution of water with baking soda dissolved in it. I usually put enough baking soda in the water that some does not dissolve, so that solution is saturated to start. That seems to keep the brass from tarnishing later. Of course, I rinse with tap water as a last step.