My brother has considerable experience in converting new shotguns into rifles. It's all very logical if one sticks with what has always worked.

1: Start with 4130 PAC-NOR steel rifle barrels.
2: For 12g 50-90 or 50-140, for 16g 45-120.
3: You want to duplicate Nitro for black loads only.
4: 45-70 is ok with trapdoor loads only.

You want to keep pressure at 10 tons or less. The gun will hold up.

Albeit, you won't be able to flatten a Buff with one of these you sure can drop anything in NA.