I assume you mean smoothbore 8? With little or no chokes, round balls can be loaded just as easily as shot.
: The ball must pass through the choke, if there is one. If no choke, no problem. A series of wads is needed to take up extra space in the case, along with protecting the ball from the powder. Friends of mine used cloth patched balls in 12 bore shells to shoot balls from their full choked duck guns in case a moose wandered by while they were shooting ducks. This happened enough for them to always pack a few round ball loads. With good fit, the balls were much more accurate, and more powerful & deeper penetration than factory slugs. Their loads duplicated the African 12 bore ctg. round ball loads of the 1880's & 90's.
: I shortened the bores of a "Fluid Steel' double underbite, cheap English gun from the early 1900's. At 26", I put a blade front sight, with a flip-up express-type sight. I also mounted a scope for load developement. After a LOT of experimentation, I had both barrels printing together at 100 yards and shooting from 6 to 8" groups, 2 lefts, 2 rights very consistantly. I figured this to be my maximum range on deer or moose, merely due to the accuracy of the smoothbore.
: I developed both smokeless and black powder loads with the smokeless loads producing the same velocities as the black powder ones, and duplicating the loads used in the BP 12 bore black powder ctgs. for the early round ball rifles.
: I see no reason why you couldn't do this with an 8 or 10 bore as well. Those 545 gr. balls I shot did pack quite a whallop. The big bores would be devestating on game.
: Forsyth noted that some large bored ML'ers for the mid 1800's were rifled too fast to take heavy loads for tiger, buffalo and elephant, so the owners merely loaded them appropriately for the game involved and limited themselves to smoothbore range as the balls stripped in their very fast twists, giving no better accuracy than a shotgun loaded with ball.
; You can order a proper sized mould (you tell him the size you want according to your bore and choke size) form Jeff Tanner of England. A google search will give you his web site. The mould quality is good and I have 2 of them, ordered separately. I think they're still less than $30.00 which is pretty cheap for a custom mould diameter made to your specs. An 8 bore would work for everything, anywhere, as long as you develop good loads and don't overextend your accuracy range.