In the earlyish 19th century the British Ordnance invested large sums for the day in machine swaged ball making machinery as they eliminated the inevitable void in the cast ball below the sprue. As a guide it made the ball about 6% or so heavier for the same diameter. and made them so much faster that the savings in labour over time paid for the investment as well as getting better balls.
By the 1820's I think it was, the switch was made to swaged balls in the States and the ball's size was increased from .64" to .65" for the .69" muskets.
This lead to doubling the hits expected at 100 yards, on the target the military used, turkey targets. They went from 1 hit in 5 shots, to 2 hits in 5 shots,
just by increasing the ball size by .010". These were for their issue paper ctg.'s of course.
I suspect most of the guys here have the same results as I do, and any ball that had a void, would be returned to the pot. Balls I have cast and sectioned,
had no void under the sprue. They are also a LOT more consistent than the commercial swaged balls available today.
Later on, in the ctg. era, large ammunition companies like Sharps and Winchester also went to swaged bullets for paper patching. They hired women to patch
the conical balls.
Grooved bullets for outside lubrication, of course, were still cast.