Wash the brass in a mix of a liter of water, a table spoon of liquid laundry detergent and with two cups of cheap white vinegar. It'll quickly remove all but the worst possible tarnish from the brass and leave it looking like you hand buffed ever casing. Just dump it in and agitate a few times over the course of a 10 minute soak and you're done.
Using this same mixture in an ultrasonic cleaner on brass that has been de-capped leaves the inside looking like it was new again.
In each case rinse well with a first rinse bucket and then transfer to a second rinse to remove the soap and vinegar from inside and out.