I found with my Kodiak, that increasing the powder charge, not only uncrossed the balls, but made them shoot level as well.

I am glad the slugs shoot too poorly to repeat, and that you will be shooting round balls. I would be concerned about losing my left hand when the left barrel blows. It will probably happen, sooner or later - BLAM!

In the 19 century, slugs did not become popular for heavy game until after breech loaders were developed. thus, with crimping (just look at the crimps they actually used!) so the slug was sure to stay on the powder due to that haeavy crimping and the bullets themselves being oversize to the rifling, could not migrate up the bore.

In India as well as Africa, the large bore round balls, if hardened were found to do just fine. Indeed, with only 4 1/2 to 5 drams of powder, a hardened 14 bore ball (merely 470gr.) would penetrate "Through and through an elephant's head" - THAT was Samuel Baker, by the by (way).

A 1 1/8 ounce (492gr.) (13 bore hardened) or more round ball, hardened or well over 500gr. if soft, will kill any game to be found on this continent and the hard ones, all in Africa, as well.
Tom -
My Kodiak .58 crossed badly at 50 yards - by 2" and shot to different elevations, LEFT HIGH, RIGHT LOW both off by 3" difference as well, when using a mere 80gr. 2F. Knowing this, I was able, offhand to put a right and left into the bullseye at 50 yards. shooting offhand but that 'trick' or 'dodge' used by some gun makers of the 1800's according to Forsyth, was only that, a scam to make the barrels appear to be regulated properly.

I would not want to have to think about where the balls actually were going to strike at whatever range the game was and hold off for that error. YUKO!

Increasing the charge to 100gr. 2F GOEX, both barrels, rights then lefts, shot into a 1" group off the bags, ie: crossing to shoot together, held as I do when shooting off the bench with guns to make the impact the same as offhand, gripping the forend or barrel or barrels with my left hand, resting the back of that hand on the bag, at 50yards - no rear bag.

Increasing the charge further to 110gr., the barrels shot parallel - perfect regulation. I've shot this rifle at long range, out to 250yards in the summer, fall and at 100yards in the winter and the single sight hits centre, left or right - I love shooting trail walks with this rifle - a right-clang, then a quick left-clang again. it's lots of fun, but hard on the powder and lead & cap supply. It's easy to go through 80 to 100 shots in a single day on the range, no wiping is needed at any time. More powder yet, is needed to regulate my rifle, with an oil patch.

Increasing my load further to 120gr., the balls crossed again, except opposite, left shooting low, and right shooting high by 3" separation and 2 " crossed at 50yards. indivisual groups were still less than 1"./ It's an accurate rifle in spite of what looks like a total lack lapping (crossways reamer marks) by the maker- Pedersoli. By now the wear is longitudinal and the gun shoots cleanly and very accurately.

So - the only way to find out what's going on, is to shoot it - with round balls and patches. Neetsfoot oil (real, not compound) works for hunting patches and final regulation for hunting. Be sure to record all your work on the rifle- and each tube.

It will usually take a bit more powder when using oil (for lube) to regulate than when shooting with a water based lube - The difference is usually a good 10 grs. more.

This is a perfect environment when using 110gr; for plinking and 120 to 140 for hunting - in any of them, .58 or larger.

In the Kodiak .72, a .69" round ball (Lee or Lyman mould) could be cast from straight WW or 20:1 or evern 16:1 alloy and penetrate everything you wish, on either continent.
Best wishes