For a 16 bore's shot load, I'd start with 60gr.2F and single card with fibre cushion wad, then 1oz. shot, then thin "B"card top wad.
For a round ball load, I'd start at 3 drams 2F - approx. 82gr., and increase from there - probably to 100gr. 2F if it shoots well. A .640" ball would be the largest I'd shoot- probably get a Tanner .635" mould. I would prefer to stay with a ball of 400gr. or over, - but? I've found that in a smoothbore, a ball .025" to .030" is a good size. This allows a substancial patch that holds lots of lube for cleaner shooting, shot to shot. Mink oil has worked fine for me for hunting when only a few shots are fired - maybe up to 10 or so with heavy charges - in the 16 bore smoothie, you probably won't be shooting heavy charges so mink oil would allow more shots. Something like neetsfoot oil (not compound) mixed with olive oil, beeswax or just straight would work too.