I do not think the under bored barrels are what are causing your tight patterns but rather the points of constriction combined with plastic shot cups. You could try fiber wads to see what results those give. If you look at enough choke constriction charts you will soon see that not everyone agrees on how much constriction will give you M/F patterns. As a general rule of thumb, I consider plastic shot cups equivalent of adding .010" of constriction. In other worlds, opening things up .010" should get you close to what you are looking for. Best to go slow though, much easier to remove constriction as opposed to adding it.

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