Yo Butch -
Yes, I've had some zinc cannon balls cast - but all were too large and when casting zinc that large shrinkage is irregular. I WILL make some on the lathe from CRS this summer. (Have to wire the shop, hook up the 3phase converter etc. beforehand.)
Almost no problems with concrete filled PVC pipe - the concrete doesn't compress and hence is dimensionally stable when firing while the PVC pipe is precise and fits the bore VERY well (4.550 bore with 4.500 boolet). The only problems are that one has to wipe the bore after each or every third shot to limit fouling buildup and once the concrete hadn't set thoroghly so it slid out of the PVC pipe - both were lanuched).
I spent 6 months at Ft Sill - Field Arty OCS in '72. Fun time shooting 105's and 155's.