My initial two reloadable PVC Motors were not too successful.

Blow By

The threaded PVC Couplers had some gas escape through the threads. I had just sprayed them with some WD-40 to make them easier to open after firing, but a heavy grease would be better.

Shrinkage

The PVC pipe's ID was smaller after firing. I did not measure before, but the grains would no longer fit.