Experimented with a few cases last night.
Mixed up some Gatorade powder into hot water and put in 2 black cases.
Mixed up some Fruit Fresh (wife had it in the kitchen) powder and put in 2 black cases.
There was no control for strength of solution since I don't have anything to test pH with.
After 90 minutes the Gatorade solution was showing lightening of the tarnish.
FF - no sign. So I dumped in some more FF.
Next hour saw progress in the FF solution and the Gatorade kept going.
Left them overnight. All cases were "clean" except for one of the Gatorade ones which still had a dark side. I attribute this to not agitating the solution overnight.
Of concern is that the brass, while no longer tarnished, is dull looking and a coppery color much like the Makarov brass I've seen discarded on ranges. I plan to drop the cases in the vibrator (mix of corncob and walnut media) and see if they polish up to normal color.