Earlier, "Charles S" wrote:

...My question involves how the
Germans in WWII fired cannon through the
propeller hub of V-12 engines.
I cannot see how this is accomplished,
since to my knowledge there were no
gear reduction units on these planes..

There were, in fact, gear reduction units on the DB601 and similar
engines. The cannon was oriented through a tunnel between the two
cylinder banks. The tunnel opened through a bore in the center of the
drive hub of the propeller.

This page pretty much describes the system, and has a diagram that
shows the cannon tunnel:

http://www.aviation-history.com/engines/db605.htm

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com