I tumbled some old lake city 308 I got from an army buddy. It had been in a belt at some point and was very dark with tarnish. I plopped a hundred rounds at a time into my tumbler and ran it for 24 hours each batch (swapping batches while home for lunch) they cleaned up REALLY well. No belt link marks, no tarnish, just nice clean brass. The copper jacketed bullet turned a pinkish red copper color rather than the orangey color I expected but it shot plenty accurate. Gave all that ammo away as christmas presents. I have since ran a lot of milsurp 5.56 and 45acp through just to clean it up. Never had a thing happen that I would consider to be "bad" in fact one of the best things about tumbling loaded ammo as a reloaded is that it checks neck tension for you. If neck tension isn't acceptable you may not know it by your loading process, especially when loading high volumes on a progressive. Loose necks allow bullet setback inside the tumbler and it's really easy to pull the rounds apart and apply a better crimp. Found this out with old reloads in .270 win.