If the bore is a true 12 gauge, it will measure .729" to .730".
A .700" ball will be .030" smaller and will allow a thick, nicely lubed patch, such as .020" with easy loading.
Some like them tighter, some looser.
.690" is a standard size by Lyman and Lee, with roughly $100.00 difference in the price of the moulds.
Tanner Moulds made of brass in the UK, are in between those two price wise.
Tanner Moulds will make any size you want & are not restricted to "standard" sizing.
Their moulds fit Lyman and RCBS handles and perhaps the newer Lee handles as well.
I own Tanner moulds in .595", .677", .700", .724" & .740". They work very well for me.
A simple cut-in and twist of the wrist or ball leaves a perfectly round, smooth ball without any sprue that needs orienting.