to all: thanks for all the input and discussion. We scrapped the PVC tanks and valves (kept PVC barrels), went to steel 1/2 gallon air tanks (holed sawed a hole in one end to remove the 3/8" fitting and had a 3/4" or 1" bung heliarc'd in). Valves are 12V metal valves (used for clearing the bags in dust collection systems?).
MrBasse shared a photo album with *their* shooter, and gave permission for me to link to the album. They have "cartridges" that they can preload...
https://drive.google.com/folderview?...&usp=sha ring
I'll post pictures of where we ended up after scrapping the PVC tanks and valves.