What wittzo said.
I've rolled my own paper shot carts for the Howdah using that white paper that Cabelas loves to use as packing material.
First, I make it into a paper tube with one end twisted or folded/crimped over.
I then add my measured powder charge.
Then, I twist the powder filled end closed and add my shot charge.
Then, I fold/crimp/glue it shut.
To load, I tear open the powder end and dump the powder into the barrel.
Then, I ram the rest of it down the barrel.
I've also made paper tubes with the end twisted or folded/crimped over, and just filled them with shot and twist or fold/crimp the other end.
In this case, I load the powder from a powder measure, straight into the barrels.
Then, I ram down the paper shot cartridges.
Either one seems to work as well as anything.
I do need to nitrate the paper, though. It sucks having to walk around stomping out flaming paper after shooting.
Or maybe you could get some of those blunt wrappers I see the hood-rats buying at the gas stations.
Instead of filling them with stupid-weed, you could make paper carts from them.
Or, you could buy flash paper.