You can always "Lap" out a mould to increase the diameter of the bullet it casts. It is a fairly simple procedure:
[1] For a Minie Ball Mould: Move Sprue Plate a quarter of an inch or so, so the hole in it no longer alines with the pour end of the mould. You want that end of the mould "Closed."
[2] Flip the mould upside down and put the Base Pin aside. If your mould has a "Floating Base Pin" arrangement, then remove the entire Floating Base Pin Assembly.
[3] Suspend an old flat bladed screwdriver in the mould-MAKING SURE THAT NO PART OF THE SCREWDRIVER CONTACTS THE INTERNAL CAVITY OF THE MOULD.
[4] Then pour in your hot lead into the bullet cavity and let it harden,
[5] Open mould. What you will have is a "Minie bullet" with no base cavity and a screwdriver sticking out of the base of the Minie bullet.
[6] Coat the Minie bullet with "FINE" grit VALVE GRINDING COMPOUND obtained from your local Auto Parts Store.
[7] Re-insert Minie bullet back into the mould. Put mould in jaws of vise to hold it closed (but not too tightly at first.) Then rotate the Minie bullet in the mould. The Valve Grinding Compound will literally "sand" the sides of the bullet cavity and enlarge it.
[8] As the size of your bullet cavity increases, you will have to repeat Steps [1] through [7] several times.
Just go slow and often check the size of the Minie bullet your mould is casting in the muzzle of your rifle as you don't want to enlarge the bullet cavity of your mould too much!
GOOD LUCK!