They may have used it that way but I understand that a true grapeshot had few larger balls compared to canister that had more but smaller balls. Because of that grapehspot was also used at larger distances compared to canister. While grapeshot was also effective against persons it also served to cut rigging, destroy spars and blocks, and puncture sails. Canister afaik was pure anti-infatry at short ranges.
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Lieutenant General Christian Hecht
Commander I Corps, Army of the Potomac
"Where to stop? I don't know. At Hell, I expect."