I don't think you will live long enough to polish shells with a rotisary motor.
My home made tumbler is running around 700 rpms. It looks slow even at that speed. It has done a really good job for me since I built it about 25 years ago.
I used a 1725 rpm motor and redused the speed through pulley size to 700. I put a 2"pully on the motor and a 5.25 on the first drive shaft to get this speed. It may be a little fast but it still takes around 2 hours to fully polish a load of 9mm shells.