The "nominal" bore size for a true 8 bore is .835". A pure lead ball that size, weighs 875gr.

It was common to use one bore size smaller ball, which would be 9 bore, at .803" with a substantial patch. A 9 bore pure lead ball weighs 777.8gr.

The pressure for whatever charge you could handle, will be extremely low. I would estimate never over about 6,000 to 8,000 psi at 1,200fps to 1,300fps - for whatever ball diameter you choose.

I had a late 19th century SxS 12 bore, stamped with "Whitworth Fluid Steel" barrels. I assume it is hot cast, then bored. They were nitro proofed.

Keep up "posted", ironoxide.

For an .830" smooth bore, I would most likely use the 9 bore mould to allow a thick patch and lots of lube. You should be able to load a ball that size and .018" denim patch quite easily.

The bore might need a scrubbing with scotch-brite cloth and oil. This is an abrasive thick plastic conglomerate that really smooths minor problems out well.

I used it on a ladies' rifle barrel at the rendezvous last week and now she's back to shooting the whole course of fire without having to wipe the bore, not even once. With a decent patch, you wipe the bore as you load. Before, it had a 'rough' spot just about under the sights that prevented her from pushing the ball past after only 2 shots. This roughness happened due to poor cleaning practices - using commercial shitty products.

I made about 30 passes with the scotch-brite, lubed with WD40, full length of the bore and this smoothed it up.