My own Kodiak crossed with 80gr.2F, the right shoting high, left low but with 100gr.2F, both tubes shot into the same group at 50 yards, crossing by an inch and a bit at 100.

With 110gr. 2f shot they perfectly parallel, 1 1/8" apart, then with 120gr. they crossed again, only this time, the right shot low and the left shot high.

Not only changing the load, but changing patch lube will also effect accuracy and regulation as well as might, the addition of wads, etc. Changing multiple loading components might get the rifle behaving.

I would exhaust the variables with round balls, before attempting a slug. Slugs will NOT improve it's killing properties.