I took the OP's question to mean he has a barrel that isn't a rifled slug barrel.

As to choke, we've always been told to use an open choke like cylinder bore for slugs or 00 buck. But I've found the smaller buckshot like #4 responds well to some choke, I got better (denser) patterns with #4 buck when using modified than when I used Skeet #1, which is almost as open as a cylinder bore choke.

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