Well Allen, now you've dunnit! Those Korean cannons looked a lot like line cannons to me, so now I've decided that I'll have to make up some 2.5-inch shaft finned darts and go out and play with 'em. That's all I needed was another project on top of the 574 already on the list!
Hmmm, maybe PVC pipe with a plywood sabot to cover the rear end, something inside to stiffen them, plexiglass wings if they won't shear off on firing, something to weight the nose.....
I'll break out the books and have another look at WWI spigot mortar projectile designs, might get some ideas there...