.648" ball, most likely, with 60 to 90gr. 2F would work, whatever you like. Once fellow up here in the bush is unbeatable with his 20 bore smoothbore & he shots 85gr. 3F with a .020" patched .595" RB.
.648 (.650" is 17 bore) is a standard size mould, seems to me and should work just fine in that 16 bore. track also sells .648's.