http://www.castbulletassoc.org/view_user.php?id=6317>sharps4590:
I believe the Kodiak Double has 1:48 twist bores. A 1:24 would stabilize the heavy slugs you want to shoot much better. Heavy bullets need a fast twist, light weight round balls need a slow 1:60 twist
The Lyman Great Plains rifle I have came with a 1:60 round ball twist when I originally got it many years ago, but 4 years ago Midway had a sale and I got a 1:48 twist barrel for the rifle.
I had already known the 1:48 is a compromise twist to allow round balls and conical slugs in the same barrel but that twist rate is too slow for heavy slugs.
The compromise I took that worked well was the lightest slug I could find. closest to a round ball as I could get in weight. That ended up being the R:E:A:L 250 grain mold from Lee.
If your double rifle has that 1:48 compromise twist , trying heavier and heavier bullets is likely to yield less and less stabilization of the bullets.
My 250s shoot much tighter groups than the 320s that I tried with any charge. You have hoped that a heavier slug would stabilize better, If you have the 1:48 twist, it is the opposite of that, lighter bullets will stabilize better.
If you want to try some 250s, PM me. I would send you 50 in pure lead and tumble lubed just for the shipping. They group 1 inch at 50 yards with 70 gr 3F Pyrodex in my 1:48 twist Lyman all day long. I now only shoot 3F Pioneer Powder because of my Asthma and groups are 1.5” at 50 yards. I miss the Pyrodex for better accuracy. Incidentally, My rifle doesn't like 2F coarser powder of any kind.
Let me also say, I use a trick at the range that has made my field accuracy improve. Consistent bullet seating pressure is paramount to accuracy in muzzle loading. My range ML shooting always has me using an analog bathroom scale during loading. I seat bullets with the rifle butt on the scale to a pressure of 80 pounds every time. This helps get me in the habit of using that seating pressure in the field.
Gary