Hey Pete, just out of curiosity, what do your 12-ga balls weigh.
You can calculate the weight of a lead roundball by cubing its diameter in inches and then multiplying by 1503 to get grains.
OR, you can just rough figure it using the same method the old timers used to name the bore sizes. A 12gauge gun means the bore size is the diameter of a lead ball that weighs 1/12th of a pound. (which my calculator says is 583.3333gr.
)
My Grandfather loaded his own 12ga shells from the early 1900s through the 1940s. I still have some of the rounds he loaded. Some of them are what he called "pumpkin balls" (single round balls). He seemed to put great stock in them, I doubt he ever used any of the commercial slugs. He didn't deer hunt, was a farmer, and if he ever needed something more than shot, the pumpkin balls did the trick.
Also, my grandfather's pumpkin ball loads weren't exactly just cast round balls. (he didn't cast), they were actual musket balls he had found as a youth. He lived near several French & Indian war and Revolutionary war battlefields, and collected a number of musket balls, a few cannon balls, and a number of stone arrowheads in his youth. I have some of that collection, including a few of the musket balls.
The .72 cal ball for the Brown Bess works tolerably well in a 12ga, and even the .69 cal balls can be somewhat useful.
NO, not as accurate, or as longer ranged as modern slugs. But they do give a good solid "thwop" on things at the ranges you can hit them (40-50yds or so, depending on things).