Fine and dandy, for the 10 in a low pressure load. Squeeze it into a 20 or 28 gauge loading it to standard velocity and pressure for modern loads for say an automatic and this may not be true. A 20 gauge barrel in a vintage gun never approaches the thickness of a 12 or 10. The question was a general one, not specifying gauge, velocity, or pressure of the load to switch from lead to bismuth.
So why monkey with it when bismuth loading data is readily available? Bismuth is harder but weighs less, so there will be more shot in the column to reach the same weight thus probably requiring a new deeper shot cup wad in the smaller gauge so now one has changed not only the shot thus hardness, but also the wad used. It will not require a filler in the shot cup but will take up more room. So we are not using the original lead data.
But if one insists, they could always send it off for testing like you did.
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