.319 might be on the small side. You might wait and mic the chambers and see what those are before you get a mold. Some guys will use up to a .330 round ball. I use a .323 in my original, granted its 157 years old. Ideally it should shave a small ring of lead when you load it into the chamber.
Depending on who by (and when) a reproduction .31 revolver was made, you can have .31 bore, .32 grooves and .31 chambers. Or .32 chambers. Just all depends on what it is in your hands.
A .31 really can benefit from using the biggest ball practical, including bevels on the chamber mouths. Shot one in the 70's that loved these things.