Hello Crofty
'Cartridge' in this context does not mean the modern metallic cased round of ammunition but a paper wrapped musket ball and measured charge of gunpowder.
In essence the cartridge was ripped open and the powder poured down the barrel, followed by the musket ball and the paper was forced home and used as wadding to keep the powder and ball in place (probably an overly-simplified explanation!).
The thick paper used was called cartridge paper, a term which still exists for a particular grade of paper today.
Regards
Roger
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