Ah, the old rusty shot problem surfaces again! Try this cheap,tested
method to get the rust out.
Drain and clean your tumbler barrel. Wash your shot in clean water
and let it drain. Go out and buy two cans of regular Coca Cola (or
Coke). Open one and let it sit until the fizz is out of it. Then pour
the desired amount into the tumbler, add the shot,, and let the
tumbler run for an hour or two. Drain and flush as usual. If all the
rust isn't out, repeat the process. Your shot may be a little darker
when you empty it out and wash it, but it should be rust-free, and
will still tumble your metal just fine.
If you can succeed on one can, drink the other one and have a ball!
(Sometimes they send a defective batch of soap powder, I don't know