I wonder how a .69 caliber minnie would shoot if you paper patched it? Lyman I think it is makes a .69 caliber minnie which is .685 diameter and weighs 730 grains. There are also molds to make 12 gauge shotgun slugs which are 475 grain and 525 grains. Also if the .690 diameter bullet is too small then I would look at a .715 diameter roundball and shoot it patched.
Midsouth Shooters Supply..
015-2654112
015-2654012
mold # 2654569
http://www.midsouthshooterssupply.com/item.asp?sku=000152654569
http://www.midsouthshooterssupply.com/item.asp?sku=015-2654112+&fromsearch=yes&submit=GO
http://www.midsouthshooterssupply.com/item.asp?sku=015-2654012+&fromsearch=yes&submit=GO