heree's a picture showing the loading as I suggested. As no one makes, to my knowledge, a proper shaped 'slug' for 16 bore smoothbores, round balls seem to be the answer. A flat based slug will not work in a smoothbore, unless it has a fairly long wad column attahced to it, to stear it straight ie: the shuttle-cock effect.

Note the wad colum system I employ with smokeless loads. With smokeless loads, the space between the ball and powder must be taken up with wads. The pink wad underneath the ball in this photo has the petals cut-off from a steel shot wad, which had a perfect 'cup' for holding the undersized ball in the centre of the bore. This type of loading is necessary for shotguns with chokes and seems to work best as well in cylinder bored guns as well.

On the left is the Lyman 525gr. .680" slug for use in any 12 bore - they also make a 385gr. for 20 bore. These slugs fit in normal shot cups, which hold the slug in the centre of the bore as it travels out the tube, then release it at the muzzle. The hollow base of the slug, makes the rear ligther than the nose, which helps stability, again - shuttle-cock effect for smoothbores. In rifled bores, this slug is said to shoot into 2" and even less at 50 yards.