"Marble in a drain pipe"? I think not. A foster slug has a hollow base that flares out from the pressures astern and seals the bore pretty well. Bead sight 100 yds pie plate all day.
My handload, hand cast 7/8 oz slug load will put 20 consecutive shots in a legal size sheet of paper at 100 yds STANDING, unsupported. There are several memers here that witnessed it.
When I was forking the range full time, I could hit a small truck at 400 meters EVERYTIME with a Winchester 1 oz slug. I did it as a demo 3 times a day two days a month for two years.