Seems to me that my .58 Kodiak was 48" twist, and with about .008" rifling.

With 110gr. of 2f GOEX, mine shot parallel at 50 yards, using .574" RB and .0215" ticking patch.

With 100gr. 2F, it shot into the same group, so were slightly crossing. Below this, they crossed badly and above 110gr. they the crossed again.

The barrel needed re-crowning as received, the crowns were too sharp for patched round ball loading.

I worked on loads for mine until I found a couple (100gr. and 110gr.) that shot close enough to use on set of sights only.

I never tried slugs (Lee R.E.A.L. and Lyman HB Minnies), however know a couple guys who tried them, never getting any slugs within 12" of one another at 50yards.

I would be concerned about slugs moving under recoil of the first barrel, then the one that moved forward becoming a bore obstruction when fired. The REAL bullet might engrave hard enough to stay put, for one shot at least.

I used patched round balls only.

I think it would have been fun to play with the 12 bore SXS- maybe 10 years ago. Even my little 14 bore is too 'wearing' to plink with, any more.