The "theoretical" ideal situation for a revolver is the throats being .0005" bigger than the groove diameter of the barrel. I suspect that modern revolver throats are sized for jacketed bullets today and often need reamed if you want to get good cast bullet performance.

A lot of .45 Colt guns way back in the day had groove diameters all over the place with different specs for each manufacturer. They eventually kind of standardized and since SAAMI came along and barrel making is vastly better. It's not so much a guess anymore.

One thing that a gas check might give over coating is a more uniform base, but if your mold is filling well and cutting a relatively clean sprue I don't think gas checks are worth the effort. Coating is the better way to go.