-----------snip------------

Wasn't the gub'ment, George, but Bell Aircraft.
P-39 not only had a drive shaft but a cannon firing thru the psru gearbox.

layout of engine, gearbox and cannon at:
http://www.home.earthlink.net/~tp-1/p392.jpg


So it *can* be done.
(The cannon firing thru the prop! )

-------------snip-----------

IIRC, the Messerschmidt ME-109 (a/k/a BF-109) was similar--except that the
long driveshaft was omitted, the engine was in the "normal" location, and
the canon was located in the valley area of the engine.

Peter