I use tin foil cartridges for my "beer can" mortar.... The tin foil helps prevent the accidental ignition of a charge when loaded if there are any smoldering embers from the previous charge....
Greybeard Outdoors has one of the best cannon/mortar forums that I have found to date...
Here is a thread that goes into a fair amount of detail for an upcoming design/casting effort:
Group buy-group pour, bronze 24 PDR Coehorn golf ball mortar
Lots of good discussion on design/pour considerations...
You don't swab between rounds?