I own a Husky rolling block, 16ga straight rifled. The groove is far larger and I use an unpatched lyman round ball at .690 which nicely fills out the grooves. It is loaded in a brass case which is the only case that will hold that ball in this caliber.
Most of my loading data was based on the lyman book in order to estimate a safe load (estimating between a .58 load and the 12ga ball loads.
That case can hold quite a bit of powder. How many grains should I use as my limit? At the very least I am using an oversized and cupped card wad as well as a fiber wads to take up the space if need be, and an over powder wad.

I am using 2F Goex.

Thanks
Tom