Actually, tumblers of this gigantic size are often used in industry for
deburring and polishing stamped metal parts, but not for much else.

Rock tumblers and the ball mills that the pyros use for the production
of black powder are generally much smaller, typically not much large
than 6-8" or so in diameter.

The commonly used drive mechanism employs a belt driven shaft and and
an undriven shaft, both covered with rubber for friction, with the
milling/tumbling vessel resting on top.

Lloyd Sponenburgh, a frequent poster to this newsgroup, has published a
book on the subject so he is the person that I would turn to for
detailed advice.

Harry C.