Square loads shoot really well, normally. That is the reason for lighter than modern (magnum) loads, for all gauges.

Some guys try to make modern magnums out of their 20, 16 and 12 bore black powder shotguns or smoothbores, but it is usually a mistake.

I see shot loads as high as 1 5/8oz in 20's and 2 1/2 ounce in 12's.

The shot string ends up being 30' long at 40 yards with no pattern density due to huge holes - a crippler only - like trying to shoot ducks with a 3" modern .410 at 30yards.