Daryl, as always, makes great points. The 8x57 in any load was a nitro load. The most often found 8X57R is an I / J bore taking the .318 diameter bullet. Cast bullets do well in them at or below 2000 FPS with proper alloy and lube. Many will have bores as large as .321. Don't think this means the .323 is ok. The case neck, after firing in the rifles chamber must easily accept pushing in a bullet of a diameter that will fit with slight finger pressure. I have used .321 bullets in my J bore bolt 1888 rifles because they will fit in the neck with enough room to allow release of the bullet. If not, very dangerous pressures can result. The grove diameters of many J bores run .319 to .321. Even with the oversize groves, the rifles usually will shoot very well. Mine do.
My mention of H777 substitute bp is in the bp rifle caliber. It is a bit more sensitive than the other bp substitutes like pyrodex. Over-compression causes pressure spikes. I fill the case with enough space for a card wad (in this case a .50) a pea size ball of SPG lube and then the bullet with no more than 1/8" compression. This gave good velocity and accuracy.
I have not seen 16ga load data for the 777 powder, and would likely use Real BP, as Daryl indicates.
These are very fun and useful arms. They suite their purpose as well today as 100 years ago.