I just fired my new 12 gauge octagon barrel on my Ithaca action. The first group was a dismal 2 inches @ 50yds from an improvised rest. It has a freebored chamber, and no forcing cone. It was designed only to shoot big cast slugs. The slugs tested were 775 Grain Truncated cone bullets, and 811Grain Whitworth style. The Truncated cone bullete were not used for the group because I had to zero the scope. It was 2 feet low, and I used all the ammo getting it close.
The bullet holes were all round, and perfectly square. The 1 in 84 twist is sufficient to stabilize the slugs. The old bore guns had twist rates of 1 in 60 to 1 in 140. I will let you know how it works out. Greg