I’d not heard of cut lead bar being used in lieu of ball. The use of “findings” in a fowling piece or a blunderbuss, sure, but not this. Interesting.
If you can melt lead or a similar metal or alloy (and who can’t?) and pour it into a slot between some boards, you have buckshot for your scattergun, or bullets for your “smooth rifle”.
I wouldn’t try it on the line at Boomershoot though. Well OK I might, but I wouldn’t expect any detonations, much less hits, from 400 yards.