I asked Jeff Tanner to cut me a mould for a .677" round ball. That is a true 15 bore ball size. They come out of the mould in soft lead at .675".
If you ordered a mold at .674, you would probably end up at .672 pure or .673 in 20:1 mix.
These moulds fit standard handles and cost about $35.00US payable to an account in California.
I have 5 of his moulds ranging from .595" to .740" and they only took about 7 business days to arrive on the West coast of Canada.