Pins are used to tumble in water mixed with some additives. That method produces very bright, very shiny, very clean brass when used in a water-holding tumbler. Drawbacks: you have to separate out the pins, a bit harder to do than with walnut or corn. You have to dry the brass. And it's a good idea to be there when the tumbling's done to avoid cosmetic or other damage to brass from the chemically active solution. Finally, sometimes the brass gets so clean neck tension is changed and it is more difficult to resize and then seat bullets.

If you got "ball bearings" as part of a reloading cache. odds are extremely good they are not "tumbling media" at all.