The .58 Kodiak I had, put both barrels into the same group at 50yards, using 100gr. 2F GOEX and .574" round balls with 10 ounce denim patches I measured at .0225" as well as with a ticking material I measured at .021".
It shot equally well with a .562" pure lead ball and both of those patches.
It shot parallel using 110gr. 2F GOEX.
It shot poorly with less as well as more powder than these four load combinations.
Some guys have not been able to get these rifles to regulate.
Now, whether than it due to them wanting to shoot only slugs, I do not know.
Mine did it with round balls.
Slugs other than Lee R.E.A.L. bullets should not be used, as there is nothing to hold them onto the powder when the other barrel is fired. If the unfired barrel's slug moves off the powder, it becomes a obstruction, not a projectile. This was the main reason slugs did not gain popularity until the advent of breech loaders, in double rifles.