I have not used an ultra sonic for cases. I have worked at a place that used an Ultrasonic cleaner for aluminum parts after they were run trough a lathe. They had to be perfectly clean with no residue what so ever. The parts were small motor housings for computer hard drives that were turned out of bar stock on lathes in house and the air gauged for precise diameters. I have no doubt that they will do a good job if you use the correct media. I have been tempted by them, but I have resisted. Simply because what I have works and want to spend my money on other things right now. I have a tumbler and a vibrating cleaner. I now have Stainless Steel medial for my tumbler that runs wet with Lemi-Shine. It seems to work well. It was a little pricey to start with, but you have little expense after the inital lay out. You can buy the whole kit, but I already had the tumbler so i just bought the media and got the Lemi-Shine at the local Walmart.
Good Luck and Good Shooting