Mitraille is grapeshot, according to my Collins-Robert dictionary, while boite a mitraille is canister or case shot. So, as I said, semantics.
Mitraille decharge has the sense of ‘a volley of shots’. Mitrailleuese – machinegun, is one of those words that the Academie Francaise despise, a neologism, derived from the above.. 🙂
I’d translate ‘le douze pouvant faire usage de mitraille’ as ‘the twelve [pounder?] was able to fire grapeshot’.
"I go online sometimes, but everyone's spelling is really bad. It's... depressing."