Origin of the word "slug"

Can anyone give me a source for the origins of the word “slug” meaning bullet.
I can remember reading years ago the term came from the fact that while round balls were cast elongated bullets were not because the moulds were too expensive. Instead they tended to be made in hand swagers that had to be hit with a hammer. Slugs were better than balls because they were heavier.
Can anyone validate this? Also a date would be useful and some reference in writing would be even better.
I think my original source was Narramore but I can’t be sure.